how to change hard drive

The 2012 and newer iMac is manufactured as a sealed unit and it is not advised to open the computer. Changing the hard drive of one of these iMacs requires a particular skill set and appropriate tools, and carries with it the high probability of doing damage to a number of fragile parts that would render the Mac unusable, including the display screen.

Your best option for upgrading the storage of your iMac will be to add an external SSD to the Mac, format and install the macOS on it and use that as the startup drive. It's relatively inexpensive, it's easy, risks no damage to the Mac and is pretty much guaranteed to improve the performance of the computer.

I and other veteran users here in the Community will often recommend external SSDs that are available here > HDD & SSD at OWC/Macsales.com Macsales.com has a solid focus on Mac accessories and support.

And if you're interested to know just how involved it would be to swap the internal drive, that site does provide video guidance.

Keep in mind those videos always make it look easy. You don't have a lot of room to see or work when connecting/disconnecting several cables in order to remove the big heavy Display Assembly. The cables & connectors are very delicate and easily damaged. Plus you need to be very careful handling the Display Assembly making sure to keep it vertical so that the weight of the Display Assembly doesn't crack under its own weight.