They have guides for every Mac model and sell all the parts you need. Rather than walk you through every possible hard drive replacement, if you’ve decided to go it alone, I’m going to hand you off to our friends at iFixit.
RELATED: Should You Repair Your Own Phone or Laptop? If you’re not sure you have the technical chops to do it right, you should consider asking a more qualified friend to help, or even going to the professionals. The Mac Pro is designed to have its hard drive replaced easily, while an iMac requires you to remove the entire screen. While it is possible to replace the hard drive on any Mac not listed above, how difficult it is varies wildly with the model. This may change if a third-party manufacturer manages to create a compatible hard drive, but for the time being you’ll need to go to an Apple Store or an Apple Authorized Service Provider if you need your hard drive replaced.