The upgrade module in Sys Admin will only show you the updates for the track that your on. To upgrade your track you'll need to use an update script. The script to upgrade from 6 to 10 is on the bottom of this page: http://wiki.freepbx.org/display/PPS/FreePBX-Distro-6.12.65
You'll want FreePBX Distro 6.12.65-100