Problem with phone after update

This is to be seen as a starting point more than anything. But, you will be able to sort out most little “kinks” this way. This covers HTC One M8 (and M7 etc). Pretty much ANY phone since the mechanism around the update is the same.

This can happen on pretty much any phone, from any brand. It’s not a sign that the phone or the update is “bad”. Sometimes it’s bugs, but those you actually rarely see on the unlocked/non-branded HTC phones (but it does happen). Covered it before HERE

Thing that you should start with, as prepping for the rest.
Do a soft reset on your phone, or just restart it. I think that a soft reset cleans out a little, it’s a very good place to start. There is things that will be fixed now. Phones like One X and One, just press power button for around 10 seconds with phone on and active. For M8 and One Mini 2, you need to press VOL UP button together with power button for around 10 seconds.

Battery draining more than it should.
In this order: Soft reset > Wipe cache partition > Factory reset.  First two might be enough.

Phone acting all crazy
Factory reset AND wipe cache partition.

Phone is almost crazy, just a bit loco at times.
In this order: Soft reset > Wipe cache partition > Factory reset.

Something else
In this order: Soft reset > Wipe cache partition > Factory reset.

Wipe cache partition? That sounds dangerous. Will it wipe my phone? Nah, it will will not wipe your phone. But, if a phone is unstable and loco: it doesn’t hurt to backup things that’s important.