You have a ducted system. The A/C is attached to the roof using lag bolts and there is a seal between the A/C and the roof. Remove power from the A/C, remove the cowling and disconnect electrical connections, unbolt the A/C unit and remove old unit. Then you can remove the old seal and clean roof to prepare for the new seal. Mount the new A/C onto the new seal, bolt down using old bolts and holes, then restore electrical connections. The hardest part will be getting the new A/C onto the roof and the old one down. Be sure to examine DIP switch settings on the old control board and match them on the new board.