What is the best way to decline an interview after applying for it?
Title
Name
Address

Dear University B -

Thank you very much for notifying me that I am on the short list for position W at University B and for scheduling me for a first-round interview on February X, 2015.

I have recently been notified by University A that I am a finalist for a position at Lab A and they want to schedule a second-round interview, also on February X, 2015.

As the Lab A request is for a second-round interview, I feel that I must prioritize this against first-rounds, even though I am still very much interested in pursuing the position at your lab. 

Would it be possible for us to reschedule my interview for Feb (X-7)?  I understand that this may raise logistical concerns and appreciate your consideration.

Sincerely,

User34154