Replacing the hard drive (or upgrading to an SSD) in a 27-inch iMac from 2012 to 2019 is a moderately difficult task that involves carefully separating the display from the chassis (which uses strong adhesive strips), disconnecting cables, and accessing internal components. Apple does not officially support user-replaceable storage for these models beyond RAM on the 27-inch variants, so this is a third-party/DIY procedure.
The process is largely similar across Late 2012 through 2019 models (both non-Retina and Retina 5K versions), with minor variations in adhesive handling or bracket sizes depending on the exact year/EMC number.
Skill level: Medium (not for the faint-hearted)
