(Sigh!) Yes, you should be able to EASILY change your cell. If you don't have cutoff valves on either side of it, you'll need to plug the return first and, if you have a sand filter, turn it to "CLOSED". In other words, you don't want to have a flood. BTW, 4-5 years of life for a cell is normal, so that's reasonable.

As for the lights, I can only guess. He may WELL need to replace the whole fixture--if it's leaking.

You should start reading here about pool maintenance. Go to our sister site Poolsolutions.com and read!
Also you should get a good test kit. We recommend the Taylor Technologies K-2006 and K-2006C (same, just larger amounts of test chemicals). Your pool service will hate us!