Rearrange photos in an album

You can grab a thumbnail and drag it to the place you want it to be. The others will move around to make room for the one you're moving.

In an Album the pictures can be sorted automatically by date or title:

A better way to get the order you want might be to give each picture a title in the Info Window (command-I), add a number to the title, and then sorting them by title. That way you could try other arrangements and return to the title order. Dragging the pictures to order them is hard to reproduce if they get messed up.

Here're how images can be sorted in various views within a Photos library:

Manually, as Richard described, is available in those albums which you create.

Are you asking about Photos on your iPad or on a Mac? The device given below your question is iPad, but the question is in the Photos for Mac forum?