Undergraduate changing research field with a deadline
I wouldn't sweat the summer spent.  It was a mistake but better not to extend it.  If you want a rationalization, just go with "at least the 6 weeks taught me what I don't want to do".  

All that said, if you enjoy the work, the group, etc. despite it being laser optics when your interest is NMR (or whatever) I would not feel that you need that this project determines your grad school subfield or project. People switch interests all the time when moving to Ph.D.

After all it is just a year, part time.  Doing anything is better than nothing.  And nobody will expect you to do a Ph.D. in whatever subfield you did a short undergrad project in (or "bachelor's thesis").  IOW, there is a natural breakpoint later.  Consider some people don't even have undergrad research.  So if you like the stuff and just wants to have something for the year, don't sweat it and stick with the known quantity. 

Conversely, if you hate it...bail. In terms of how to find an advisor...go beat the bushes, talk to people, look at websites, etc.  Stat.  And I would just do this exploration now, before ditching the other opportunity (so if you can't find an upgrade, you can stick with the summer topic).

[This seems like common sense, so if there is more to the situation, you need to give us those particulars.]