I've replaced backup batteries in both Dittel FSG71M and Becker
AR3201. Both are easy jobs if you know how to use a soldering pencil,
solder, and solder wick or desoldering tool in close proximity to parts
you don't want to remove or replace. You also need some small
screwdrivers. Using tiny amounts of nail polish on the screw heads when
you replace them keeps the screws in place. Don't get caught stealing
"my favorite" nail polish.
By the way, Becker repairs in the US are now only done at Becker USA
in Florida, but batteries can still be purchased elsewhere. I got one
from Erie.
Marc has a point (legality) if your aircraft is registered STD.
If you're not comfortable removing/replacing parts on a PC board,
sending the radio to the shop and getting it calibrated at the same
time isn't a bad idea.
Jim