If you re-install, Ubuntu will do the same again. Have a look at this
for an idea of what the screens you need to use look like. Note: this is just an indication.
If it was me, I'd get to the partitioning stage, and:
- select "Something Else" - you'll get a list of your disk layout
- highlight the big (~316Gig) partition (/dev/sda1, ext4)
- click "Change" (bottom left)
- amend the size to 100 Gig, and ensure the mountpoint is /
The buttons for "primary" and "beginning of this space" should be pre-selected - if not select them. Ext4 is a good choice for "Use as".
You can go with that to see if it works, or you could additionally use the now free space for /home as that guide suggests. Up to you - /home can be done later, although it can get messy. Or you can just leave the space free for now - I like to have free space for unforseen events. Your choice.