"How to fix apple error code 29"に関する回答Read the posts. You need to temporarily replace the battery with an original Apple one, connect phone to a PC, do the update through iTunes , then replace the Apple battery with yours ( if you want).
"How to fix apple error code 29"に関する回答Same issue, failed ios 9.3 update on an iphone 4s. Replaced new (non Apple ) battery with an original and then the update worked fine through itunes. Could then put other battery back in. Thanks for the tip.