How do I replace hard drive?

It depends on which of the twelve generations of MacBook you have. The twelve generations cover nine years, and have four major incompatible technical architectures:

1. A1181 - white/black plastic case, Intel Core Duo/Core2 Duo CPUs, DDR2 RAM, Intel/NVidia graphics, SuperDrive optical drive (2006-2009)
2. A1278 - aluminum Unibody case, Intel Core2 Duo CPU, DDR3 RAM, NVidia graphics, SuperDrive optical drive (2008-2009)
3. A1342 - white plastic Unibody case, Intel Core2 Duo CPUs, DDR3 RAM, NVidia graphics, SuperDrive optical drive (2009-2011)
4. A1534 - aluminum Unibody case, Retina display, Intel Core M CPU, Intel HD graphics (April 2015->)

As you can see, each architecture has its own technique for installing/removing drives.

MacBook Core Duo Hard Drive Replacement

MacBook Core 2 Duo Hard Drive Replacement

MacBook Unibody Model A1278 Hard Drive Replacement

MacBook Unibody Model A1342 Hard Drive Replacement

The new Retina MacBook (released March 2015) does not currently have a drive replacement guide, but the teardown demonstrates that it doesn't use a traditional SATA hard drive. Instead, it uses a PCIe flash module inserted into an M.2 socket on the logic board. It may be a while before replacement drives in this format are available.

So, we first have to identify your computer before anything else can happen. The model number (A11181/A1278/A1342/A1534) will be silk-screened on the bottom case. If you can boot your computer, the more-specific Model Identifier is available from System Profiler, located in your /Applications/Utilities folder. The Model Identifier, which IDs the specific generation of the computer, is listed in Hardware Overview.