Q: How to restore iTunes from backup device on a mac?

My mac book is full of memory.  So I tried to transfer iTunes files to an external hard drive(B).  This drive(B) got damaged and I had to erase it.  I have an old drive(A) that has most of my iTunes files on it from a while back.  The external drive(A) is small.  The files on my iTunes Mac do not work now because most of each file was transfered to the hard drive that is erased(B).  How can I pull from the small hard drive(A) and replace my itunes with the files from it. Thank you, hopefully that made sense.  Also, my devices are not up to date because I got lazy, and then the computer memory was too full to update.

Allee