[amsat-bb] Kenwood TM251 memory battery replacement

I have a Kenwood TM251A FM radio I use on the FM satellites, and the memory battery is going to need to be replaced. I figure sending it to Kenwood for replacement is going to cost more than the radio is worth. The only thing the manual says about it is to send it to Kenwood for replacement. Does anyone have more information on replacing it yourself? How hard of a job is it, and is the battery avaiable?

73s John AA5JG