- get 12.04 cd/usb (you were going to upgrade anyway, clean install is much better)
- start installation procedure (http://howtoubuntu.org/how-to-instal...cise-pangolin/)
- in partitioning phase pick manual config (http://howtoubuntu.org/wp-content/up...4/3-Choose.png 'something else') which means gparted.
- delete ext4 and swap partitions (destroys all data on them)
- use free space to create fresh swap and ext4 partition, ext4 should be marked as /
- continue with installation
you may have wanted to preserve existing ubuntu data on ext4 partitions, but i think it's not feasible. You could remove 1 ext4 (and redundant swaps) and make the other ext4 to expand to eat the freed space (data on deleted one will be lost either way), but resizing partitions is an annoyingly lengthy process (at least it was when i tried it last time). Removing and creating from scratch is almost instant and even with backing up to some external hdd and restoring from it, the process should be much faster than playing with resizing - but know that you have the possibility.
In case of resizing you would have to go the upgrade route.