Flexcharge Programmable Timer

Jim45D

I have a Flexcharge Programmable Timer that is losing time. Not much, but occasionally the digital display disappears, but regains itself. I've had it for almost 4 years. Question is: I know it's powered by a lithium battery. Mine is sealed on the back. Is there a special way of opening to replace the battery, or is this a must send to the mfgr. for replacement?