Disconnecting the wires from the existing thermostat may require a screwdriver to undo the terminals.
Once removed, it is important to check that the wires are in good condition so that they make good electrical contact.
Check wires have between 5/16 - 3/8in (8-10mm) exposed core. If they don’t, you may need to cut and re-strip them
If the installation guide has specified that some wires should not be connected, ensure that they are properly insulated.