Hi: The battery you need to replace is in the control panel, on the connections in the panel are 2 marked Tamp. Remove the wires from those connections and fit a wire link to the terminals. Now close the lid and try setting, if it sets now the problem is the tamper circuit. If not then it looks like new panel time.
Please replace the battery first though.
If the problem is in the circuit, you'll need a multimeter and patience to trace the problem.