You should prepare the replacement microSD with a Filesystem Image first. It's much easier and you don't need a UART connection.
Some SD-images are downloadable at downloadsnewit.co.uk/SD-image…
Remove the Power Supply Unit
Pull out the 4 rubber feet.
Unscrew DreamPlug at the 4 corners.
Pull cover left to unlock
Remove old microSD.
Put new microSD in.
Pull cover right to lock.