Dsc sensor not operating properly. I want to change it, is this very difficult? Thank you.

You don't mention what type of sensor. I assume it is a motion detector. Before replacing I would strongly suggest you first remove all power from the system by disconnecting the backup battery inside the control panel then pull the system's power transformer which should be plugged into an electrical outlet nearby. You will know that all power is removed by looking at the keypad - all lights on the display are off. Next remove the motion detector's front cover. Some detectors have a small screw on the bottom so that when removed the cover pops off. Others need to be opened using a flat blade screwdriver and prying off the cover. Once the cover is removed there should be a 4 conductor wire that is wired to a series of terminals. Observe the polarity of the wires, disconnect them and remove wall mounting screws. Install new unit. Be sure when wiring to connect power wires correctly. The 4 conductor wire should have a red (positive) wire and a black (negative) one. Connect the red to the + or pos terminal and the black to the - or neg terminal. Read the instructions that came with the new unit if in doubt. When finished plug the power transfomer back in then reconnect the battery (in this order). If power is connected properly the motion detector's red walk test light (indicating motion) should light whenever you walk in the protected area. Also make sure the system recognizes motion by first closing all protected areas such as windows and doors to get a system ready or green light indication on your keyad, then walk around the protected area. A properly working detector will show you a fault on the keypad everytime the system senses motion.