If you're using an iPhone 6 or 6+, there's a way to reboot into Recovery Mode that requires no Home button. To do it, follow these steps:
1. With the phone powered off, press and hold the Sleep/Wake button and the Volume Down button simultaneously for about 10 seconds, until you see the Apple logo.
If you have a problem where your iPhone keeps rebooting in a loop, the first thing you should do is plug it in and charge it. If it’s getting a full charge and it’s still rebooting in a loop, and if the problem only started after you updated the software, then the problem is probably software related.