When you get to the partition table by selecting "Something Else", instead of deleting the old partition(s) and adding new ones for Mint, just use the "Change" button at the bottom of the table to edit the existing partitions.
Click on a partition you want to replace and click "Change" a partition edit window will open. Leave all partition parameters the same, just click "Format the partition as..." and select your file system choice from the drop down menu. The partition will be formatted (overwriting the existing OS) and the selected Mint partition (root, /home, etc.) will be installed. * Obviously donot format your home partition.
For Grub installation, either click on the drop down menu at the bottom of the partitioning page and select the location for Grub installation, or just leave this alone and take the default installation. The default is /dev/sda. This will install Mint's Grub in the mbr over any existing bootloader (Ubuntu Grub, Windows, etc.), and set up the dual boot menu automatically.http://youtu.be/kBrRL6iG0NM
using the installer (ubiquity) after setting up your partitions. Or installing over an existing install partitions.