How to merge two references?

I have downloaded citations from several online libraries. I have several duplicates, but with slightly different data. 

Besides manually transcribing from one to the other, then deleting one of the refs, is there any feature of Endnote that I can use to do this?

I’m using X5 – just started a few days ago.  I searched the help for “merge”, but didn’t find anything about duplicate merge.

I’d like to see two windows side by side, pick one as a target, then selective copy from the donor, then delete the donor.

I don’t find anything like that described under “update”.