Replace a Hard Disk - Install an SSD
If you've got an older Macbook Pro, the non-retina version, and want to either upgrade or replace the hard disk it is pretty straightforward. This was before Apple started soldering parts into their devices, meaning that as laptops go, they're pretty hackable.
iFixit has the full details, but essentially all you'll need is a Phillips #00 screwdriver for opening the case, a T6 Torx screwdriver for unscrewing the HDD once inside, and a spudger for lifting up the hard disc and the cable that connects it to the motherboard.
Rather than simply replace a broken HDD it is also possible to install a solid state drive (SSD), which will speed up loading times.
And brilliantly, if you no longer need your optical drive (c'mon, it's 2016!), it is possible to buy a caddy and with relative ease swap it out for a second hard disk - boosting the storage available on your computer, as demonstrated in the video below.
...But before you do all of this, make sure you're able to install Mac OS on the new disc first by creating a boot USB, which can be done on any computer running Mac OS already. You can even boot directly from the USB.