Under the front of the seat you will find a bracket that holds the seat frame onto the guide rail.
Undo the bolt on the bracket and the seat will slide out..
As above, but clear out the runner channels, and give a shot of WD40 prior to removing the seat, any debris in them can cause them to jam up, and be a pig to remove, you'll also notice a nice difference when the seat is re installed, and it adjusts freely..
Thom