Battery was not holding charge
Opening case hardest part ... takes a while of prying (to the point you might not think it will ever come apart).
One of the opening tools broke doing it ... I then resorted to using guitar picks to hold case open in one spot as I pry'ed in another ... eventually it opened. You might have to use more substantial tools but just make sure you don't scratch the back of the glass (tough screen) or damage the rubber (glued under the glass).
Soldering the battery is not for beginners. A solder remover is helpful (but not a tool most will have)
The manual was excellent but could give a couple of improvements ... see "advice"
If the manual showed an exploded view of the glass/touch screen and the rubber surround one might understand the angle one needs to pry the rubber out with the glass. Maybe a drawing on where the pry is going (between case and rubber) and where you don't want it (between rubber and glass). In my case I as doing it slow enough I did not damage anything even though I did get between rubber and glass in one spot. In this spot I just pushed it back and it stuck. No damage done.
The second point is the metal cage (under the display) clips can snag on the case edges so don't just pull up ... use another tool to guide the clips.
One more issue is that when reassembling the case I just snapped the glass and rubber back into the case. I didn't try to re-glue it. I seems pretty secure. If I was to re-glue it; my guess would be to use rubber cement although the manual didn't mention anything about adding glue.