The original iPhoto events are represented by albums in Photos. To merge albums you can do it one of two ways:
1 - If you want to merge Album A into Album B do the following:
- open the sidebar with the Option+Command+S key combination.
- with the two albums visible in the sidebar click on A.
- select all of the photos in A.
- drag them onto the Album B icon in the sidebar to add them to B.
- verify all of the photos are in B.
- delete A from the sidebar.
2 - Select all of the photos in Album A and use the File ➙ New Album menu option. In the next window select iPhoto Events
and then the album name you want to add the A photos to. Lastly delete album A from the sidebar.