Editing a Memories Video

While the memory is playing, you can click the gears icon in the video controls, then change the mood and the duration.

Click the Play button on the title image to play a Memory Movie. Hold the pointer over the movie until the video playback controls appear, then click the Action pop-up menu  and choose a mood and duration.

But to change the selected photos and videos, you have to create a new memory from the items in the memory, see: View memories in Photos on Mac - Apple Support

• Create a new album, add the items from the memory to this album, arrange them as you like. Then create a new memory:

1. In the Photos app  on your Mac, select the album in the sidebar, then click Show as Memory.
2. Scroll down to the bottom of the photos, then click Add to Memories

I'd like to add something to léonie's recommendations. What I've done is add a keyword for each person to all photos that that person is in. How?

1 - using the Keyword pane, (⌘+k) add a keyword for each person in the Person view.

2 - in the Person view select a person, make sure Photos not Faces is selected.

3 - click on a photo and select all (⌘+a).

4 - in the Keyword pane click on the person's keyword to add to all of the photos.

5 - do the same for each person in the Persons view.

This screenshot will show you how to interpret the Keyword pane:

Then you can also manually add a person's keyword to newly imported photos and create Smart albums for each person: Keyword is "XXXXXX". Just another way to identify photos.