- Been there, done that.
Quite easy assuming confidence with a soldering iron. The battery (as I recall) is a standard 1/2 AA lithium cell with PC tags; the only problem I had is that the battery fitted to my ESQ1 had a single PCB pin at each end whereas the only battery I could get had a single tag at one end but a double tag at the other, so I had to do some creative snipping. The tags are a tight fit in the PCB so desoldering first will help a lot. Try not to overheat the PCB too much as this can sometime lift the tracks. You will have to remove the PCB from the keyboard and to do that you may also need to remove the keyboard itself. If in doubt unscrew everything!
Whilst the PCB is out it does no harm to re-seat all socketed chips as they tend to creep over time and cause some unreliability.
If you can't get the exact battery size, theres enough room to put an oversize cell in and use flying leads.
Just dont get it the wrong way round!