The Exit 300 button has worn out, can we repair it or there’s only one option to buy a new one?

The EXIT 300 button has 2 micro-switches, if under “worn out” you mean that the button does not provide an electrical contact, or there’s unstable contact between the red and black wires, you may try to swap around the micro-switches.
If the button’s started sticking in the case, you need a new button for exit for repair won’t help.