i also bought a used 512gb ssd from some guy in kansas.. ssd had about 1 month of hours on it. works great so far.
Pentalobe screwdriver to remove all screws underneath, then you open your computer and take the opportunity to blow dust from the fans and heat sinks with aresol dust remover , wipe down the bottom plate for dust, just unscrew the old ssd, insert new one, job done.
I used Disk Utility in Recovery to clone it via RMBP SSD sata adapter -> enclosure -> USB first however, so it was literally a question of swap and go.
old 256 GB ssd is currently powering my i7 desktop.