Use step 39-40 of Home button replacement. Be careful not to tear gasket and don't go further and remove gasket.
iPhone 4S Home Button Gasket Replacement
I also had problems getting at this screw, you really need to move to Step 27-31 of removing the headphone jack.
iPhone 4S Headphone Jack Cable Replacement
Don't recommend going any further and messing with the antenna. Then resume to step 34. Likewise ensure you back track when reassembling.