An entitled alien prince teams up with a ditzy teenage girl to help him must travel back in time to alter future events in order to save his planet.

I revised it.

Fun idea! I'd love to see descriptions of the protagonists, especially if they are an unlikely pair who are forced together because of circumstances out of their control. For example:

An exuberantly loyal alien prince and a sullen, despondent teenage girl
A desperate alien prince and a ditzy teenage valley girl
An entitled young alien prince and a poverty-stricken teenage girl

If you already have the characters flushed out, just add the appropriate descriptive words. If you don't have ideas of who the characters are yet, work on that first and it will inevitably make the logline more interesting.

Hope this is helpful :)