How do you replace the rear shocks in 1993 buick lasabra

only if their damaged or sagging

This is most likely the telescopic pistons that support the trunk lid as it rises. When these start to fail they can't hold the weight of the tailgate or deck lid. THEY ARE usually cheap to buy and easy to replace. *Note* Get a helper to hold the trunk lid open while you replace them so it doesn't slam closed on your head. I learnt that one the painful way!
If yours has the 'push button' power type the power unit could be at fault.

fuse box under hood,or rear seat

i'm not sure that you have "air " shocks or struts , follow this link to an automotive parts site , put in your vehicle details and it will show exactly the type of suspension components you require
Replacement Parts Car Parts and Truck Parts

Pretty sure your car has struts on the rear and the shock function is part of the strut. Replacing struts is more difficult than replacing shocks and they should be replaced in pairs.

Did you check the fuse? Not only check the fuse, check voltage on fuse circuit , goes hot with key on. The bottom info should be for relay center location.

Your Buick LeSabre may have air shocks on the rear. Check the shocks for leakage or stains from previous leaking. If the shocks are leaking you'll need to replace the set. Don't replace only one shock-replace them both.

Did you replace the crank sensor?

try www.autozone.com to see if they have your car listed for diagrams pictures and step by step instructions if your car is not listed then try your local library they have all the car manuals and even copy them for you or you can try www.schramauto.com or call them at 1-800-292-1032