you can see where the screws were. you need to unbolt the whole seat assembly and the sliders as a unit from the pedestal base.
you have two choices for what to remove.
View attachment 420195
then, remove from the boat, flip over on the workbench. Then re-attach / re-screw everything and lubricate the sliders.
then re-install.
based on the discoloration of the vinyl, you will most likely find the wood substrate is rotten.
Those cheesy sliders have been used on millions of boats with back to back seats. they need yearly maintenance.
on my seats, I ended up with abandoning the T-nuts and have a fabricated stainless nut plate embedded in the substrate