With the seat out you can proceed to install your replacement seat if the replacement came with good trim, a working seat belt latch, etc.

Otherwise, you may find that you need to first transfer those good parts from your old seat to the replacement seat.

To do this, you will need to remove the three trim pieces. They just pull off, though they often take some care and effort. Before removing the seat knob trim, the knob should be pulled off.

Seat knob trim

Seat bottom trim, outside edge

Seat side trim, around seatbelt