Deductions & credits

Hi - I'm married filing jointly, and I entered $300 for this the first time mistakenly during the final review process.  When I tried to change this to the actual donation amount ($500), there was not a way to update this again.  Even the donation section no longer flows through to update this number (box 12b).  Do you know if there's a way to manually update this once you've done Final Review the first time?