My iPhone 4s says it has an update ios 6.1.2 available but the download and install isn't highlighted for me to click on
If the download and install button is not highlighted, then your iPhone 4s does not have enough memory. Check your memory status and delete some of the applications that you do not need.
Dont update it
I had the same problem but I was at sheets gas station & used their wifi & it worked for whatever reason good luck.