my iphone 6s is asking me to change my passcode

Do you have an Exchange or work related account? If not there appears to be a bug affecting many. See this thread:

Re: I was just prompted to change my passcode within the next 60 minutes on my iPhone 6?? Ive never had this occur, please help!

I just got this also. It appears unrelated to Exchange accounts. I went through settings and changed it - and was forced to use six digits.