Reinstall Ubuntu on existing Partition

UEFI or BIOS?
Do you have swap already?

If you use Something Else and choose existing Ubuntu partition with change button, it will over install that partition. You still have to select ext4 & / (root). It will auto find swap if you have it. And default install of boot loader on that screen is to sda, which is correct, unless you have multiple drives.

Lots of detail, screenshots and essential info.14.04 Something Else example
http://www.dedoimedo.com/computers/u...all-guide.html