Try this :
from the 'start' menu in Lubuntu ( lower left K icon ) select 'Leave' then go up to 'Logout'. After it closes your session, you should be presented with a logon screen where you choose the user ( you or guest ).On this same screen there should be a selection box you can pull down to choose Ubuntu.
If not, there are other ways, but that is the easiest. Or, rather than muck things up ( as a newbie
) just reinstall Ubuntu ...
Good luck !