Declining a paper review after accepting it and seeing the manuscript
The best course is to just inform the editor that you are unable to perform/complete the review as it is outside your area of expertise. The title led you to think otherwise. You are sorry, but others are better qualified to judge and give advice. 

I don't see that as anything strange at all, and no one is likely to think less of you. 

What would be wrong is to give a review of something you don't comprehend completely. No one benefits from that. 

Just be honest with the editor. No one knows everything.