I know but I try to keep it clean for when the time comes for the designer to have easy job, but as I've said I kinda abandoned it, found more profitable web frameworks maybe I will return to it later.
First you need to add a JRE to eclipse (Window -> Preferences -> Java -> Installed JREs -> Add)
Then right click on your project -> Properties -> Java Build Path -> Libraries -> double click on the current JRE library -> Alternate JRE (and select the one that you have added above)
If you have already compiled the pack with another java version you will probably have to do a few more steps.
-Click on your project on package explorer.
-Go to the top left and find Source -> Clean up
-Right click on build.xml -> run as -> External Tools Configurations -> JRE -> Separate JRE -> select the one that you want to use -> Apply -> Run