Hello and welcome.
Unfortunately I have no manual, but a little experience with this model ( I have one in my collection that needed a full service).
To remove the platter you just have to pull - with a bit more force. It seems, that the platter of your turntable has never been taken off.
There is no screw or spring etc. to secure the platter in place.
If you got the platter off, make sure to clean the motor pulley and the area on the platter where the belt runs about - otherwise the residue of the old belt will cause noise while spinning. At mine I cleaned the pulley and platter with fine steel wool and polished both a little bit with car polish. After that there was no noise anymore.
If you once have the platter removed you can seize the opportunity and lubricate the
motor pivot with a few drops of oil so it runs smooth.
Hitachi PS-58 with audio-technica AT150MLX;
Cambridge Audio Azur 651A / 651C / 651T / 651P; KEF LS 50
Hitachi HT-6M, Hitachi HA-330 / FT-340 / D-230; Akai GXC-760D; argon audio 6210S;