Take off the mat. There are holes/slots on each side of the platter. Put a couple of fingers in each hole, then push down on the spindle with your thumbs while pulling up on the platter:
https://www.youtube.com/watch?v=6cwJIZG1Gps&t=63s
Help! Monolith turntable platter removal
I'm preparing to move house and wanted to pack up my Monolith turntable so I began what I thought was a simple process of disassembling the turntable. OK - it's not that simple. Can anyone guide me in removing the platter? I made the mistake of first disconnecting the drive belt, and now it's loose under the platter.
I emailed Monoprice, but don't know if their customer service is any good, so I thought I might get advice here.