iPhone 4s keeps rebooting after update.

Hi there i have an iPhone 4s and yesterday i updated my phone to IOS 9.2.1 and for some reason it keeps rebooting.It starts up and after 5-10mins it shuts down and its stuck in this loop. I have to press the home and power button down for 15 secs to start it up again. I am thinking maybe something went wrong in the update what should i do and how do i do it?

Thanks, Max