Lift up shower tray, put a few pieces of wood underneath and unscrew from below.
When fitting new tray put masking tape where the tiles will meet the tray and spray foam between tray and the corner.
Then shove the tray tight against the wall.
When the foam starts to get a little bit hard pull off the masking tape and you will get a waterproof seal and no foam on your new tray.
When fitting a new trap I always use clear silicone instead of the hairy washer, I also put silicone where the pipe goes into the trap for a better seal