Apple has a workaround for iPhones with a battery percentage indicator that won't update
With the battery percentage indicator failing to update, iPhone 6s and iPhone 6s Plus owners could think that they have more battery life on their handsets than they really have. You can imagine what the fallout would be in a situation like that. Apple does offer a workaround. Restart your iPhone, go to Settings > General > Date & Time and make sure that the setting for Set time automatically is turned on.
If you try the workaround and the problem happens again, Apple says that you should contact Apple support. Meanwhile, the company is investigating the cause of this issue and hopes to soon have a solution (which would probably be offered in a future software update).
source: Apple via AppleInsider