There are two good reasons to upgrade your hard disk drive. You may need more storage capacity or you may wish to speed up your rig by switching from a mechanical drive to a solid state drive (SSD).
Can you upgrade?
If you can physically upgrade the storage on your computer, then this is not usually a complicated thing to do and can be carried out with few tools other than a screwdriver. While desk-top computers usually have plenty of slots for extra disk drives, some slim laptops or all-in-one tablets have no provision for upgrading their storage. If the storage can be upgraded, there may be provision for only certain size or format of new storage devices, for example 2.5 inch bays or the latest M.2 format. This will restrict the type of storage that can be installed. 
Which drive works for you?
If you can upgrade, then you need to find out which sort of drive is compatible with your machine. It is worth considering what sorts of storage devices are available. Here are some of the current sorts on the market:
Mechanical hard drives
Solid state drives
SATA connected SSDs
M.2 format SSDs
Mechanical Hard Drives
These hard drives offer reliable performance at reasonable prices per gigabyte. They are much slower than SSDs and provide economical solutions to large capacity secondary storage where speed is not at a premium. They are not recommended as the main system disk where a smaller SSD will improve boot up time and system performance considerably.
Solid State Drives
These are available in two formats; the familiar 2.5 inch disk drive and the newer M.2 format stick. The SATA connected drive will fit in most desk-top computers, while the newer M.2 format requires a special slot to be available. This format is becoming available in high-end and light-weight laptop computers.
The M.2 format is effectively a miniature PCI-Express socket with a theoretical maximum bandwidth of 32 Gbits/second compared with 6 Gbits/second for SATA connected drives. M.2 format drives can hit much higher speeds than are possible over SATA. Premium M.2 format drives using NVMe technology can deliver close to four times the speed of a SATA drive.
All M.2 have standard dimensions and SATA drives are usually supplied as 2.5 inch format. Just bear in mind that there are two thicknesses of SATA drives – 7 mm or 9.5 mm. You may need to check on your laptop computer whether it is limited to the thinner size. The use of a spacer can pad the thickness of a slimmer drive in a 9.5 mm space.
Capacity or Speed?
The performance difference between an SSD and a mechanical drive is not only about data transfer speed, but also on seek time. On a mechanical hard disk the head has to physically move between different areas of the disk surface as it fetches bits of data for different programs or processes. On an SSD all locations are instantly addressable so performance is fast, smooth and responsive.
Type of Storage	Benefits	Disadvantages	Notes	
Mechanical hard disk drive	Large capacity at relatively low price per GB	Slow, not so good as system drive	Reliable, tested technology	
Solid state drive (SATA)	Fast with commonly available 2.5 inch slots	Quite expensive especially for large capacity drives	Premium storage for data centres	
Solid state drive (M.2 format)	Small but very fast 	Need specialised slot and expensive	Good for video or very large files	
Cloud storage	Unlimited storage	Not under personal control	Has an on-going cost	
A large capacity (up to several terabytes) hard disk drive is economical if speed is not essential, but economical storage is. Mechanical hard drives are available in both the larger 3.5 inch and the 2.5 inch size. For a primary drive SSD, a capacity of 256 GB or even better 512 GB would be ideal and affordable. Similarly an M.2 format drive may be used as a primary (system) disk or as super-fast secondary storage. 

So far we have only talked about internal storage. Using an external disk drive to store your data is also a possibility using any of the drive types discussed here. An advantage of this is that you have portability of your data and can take it from one computer or location to another. You could also choose to store your data on a server in the cloud and have it accessible from anywhere that you can use an internet connection.
Upgrading to a new system disk
There are several options to transferring the contents of the old disk onto the new disk. One method is to make a clean install using a version of the operating system on disk. This method will remove all existing programs you have. These will need to be reinstalled after the new disk is operating. An alternative method which will preserve your existing programs is to use cloning software (often supplied with a new disk drive) and make an exact copy of the current disk. This can be quite straightforward, but if the new disk is a smaller capacity than the one it is replacing may involve you in deciding what to keep and what to leave out. A third route is to make a virtual image of the source disk and copy it onto external media, then boot from a disk containing the imaging software choosing which files to keep.
Good luck if you decide to upgrade your storage.