It depends on how you installed Ubuntu. If you created a root partition only, you'll have to backup your data before reinstalling your OS, which could be through the First option or through "Something else" (I mean, if that's the case, you'll necessarily need to erase your data). However, If you have a /home partition where your data is stored, then you should choose "Something else" and do the proper installation steps.
It won't be too much different from the first time you installed your system. There you'll be able to pick the same partitions already in use by your Ubuntu and set them as you wish, since you'll have the choice to either format or not your /home partition (in your case, you'll have to uncheck "format" checkbox for your current /home partition).
The link below might be helpful. It carefully explains the possibilities.