I am so ****** at Apple right now it's not funny. This is my 4th iPhone and my worst upgrade experience ever. Does this company not use Beta testers anymore? So many OS problems! Considering returning my 6 and - OMG - getting an Android. Enough venting... To answer your question:
My WiFi goes from being stable for hours to - all of a sudden - not being able to maintain a WiFi connection for even a few minutes. It took hours the first few times to try to correct the problem. I finally learned that I need to do these two things:
1) Router: reboot it using its web based control software - and - here is the key: not just power cycle it! That did not help.
2) Phone: Reset Network Settings (Settings>General>Reset) Perhaps also power cycle it? I have done that but am not sure it was necessary.
After doing both at the same time the phone has held onto the WiFi connection for up to 12 hours before going glitchy.
Comment: Now that I know this relatively simple fix works at home it's fine, but if the phone can't hold onto networks where I'm a guest (at my office, in a mall...) am I supposed to find and kindly ask the network admin to re-boot their router for ME? Come on Apple.