Almost every full course I teach has at least 1 person who let it expire and unhappily is taking the full lifeguard course again.
Don’t let that happen to you. If you let your certification expire, you have to take a full lifeguard class all over again.
The Red Cross maintains an online database of certifications. That means you must keep up to date and there is no way around it.
To know what you have to do to stay up to date, first you must know what you are certified in. Upon successful completion of an American Red Cross lifeguard course, you are certified in lifeguarding, CPR, AED, and first aid.