Some time ago, I dropped my iPhone 4S and the volume up button never worked again. I was able to work around that, especially when iOS 7 came out, but it was annoying.
Having opened several i-devices before without messing anything up, I felt confident I could do so again and, with the excellent guides, this was easy for me. It turned out the volume up/down buttons have a small disk on the back of the button and the one on the volume up button had slipped slightly out of place, interfering with the button. Once I slipped it back into place and re-installed everything, the phone works flawlessly again.
Take your time. Keep track of those tiny screws and use a magnetic screwdriver with them. It helps keep them from falling where you don't want them. When in doubt, re-read the guide. Use a lighted magnifying glass; it really helps.