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?

What version of Endnote are you using?  With X5 and I X4,  you can compare duplicates and copy and paste most fields from one to the other, but not auto “merge” in the same way you can in the X5 update record facility. If Endnote doesn’t see them as duplicates, you can adjust your duplicate search to less stringent in the preferences (like only check Authors or only check title or some criteria that they are the same in) and then you can copy paste from each other. 

Othewise you can try to update the one that you like  best from the X5 update facility (from a current database) and then delete the other. 

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”.