I'd suggest you google "Toshiba <model> service manual" - pretty much every laptop I've ever owned has a service manual on line somewhere that will give you exact instructions about the method
and order in which you remove the various laptop parts. Note that the order is very important in a laptop as most components overlap and interlock, so you have to pull the right ones out first
The part you're looking for is a "left mousepad button" but as Gonz says you're likely to have to buy a full set of top plastics. Might be cheaper to just use a little spot of hot melt glue to glue the old button to the top of the contact (that little square button in the centre of the hole), but you'd have to be pretty confident to do that