how to merge photo albums on MacBook Air

What do you mean by photo albums? Are these albums in your Photos or iPhoto library? Or are you referring to two Photos libraries?

If you're referring to an album in Photos like these

then do the following to merge Album A into Album B:

1 - open the sidebar with the Option+Command+S key combination.

2 - with the two albums visible in the sidebar click on A.

3 - select all of the photos in A.

4 - drag them onto the Album B icon in the sidebar to add them to B.

5 - verify all of the photos are in B.

6 - delete A from the sidebar.

it is easiest to do with the sidebar shown (Photos View Menu ==> show sidebar) then in Photos select the source album, select all and drag the photos to the new album and then right click on the original album and delete album

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