If you already have a valid co-op work permit and wish to extend, it it is very important that you apply to extend your co-op work permit before the expiry date on your current work permit. Normally, you would apply for your co-op work permit extension in the same application as the study permit extension.
It is possible that your current co-op permit may expire before you receive a decision from IRCC on your application to extend. If you submit your application to extend before the expiry of your current study and co-op work permits, when your permits expire you will have implied status. This means you can continue studying and working according to the conditions of your permits until IRCC makes a decision on your application, as long as you remain in Canada.
If you leave Canada and return after the expiry date of your permit, but before a decision is made on your application, you lose your implied status. You must wait for a positive decision on your application to begin studying and working again. Consult with an international student advisor before making a decision to leave Canada.
If your application to extend your co-op work permit is refused, your implied status ends. You must stop working immediately.