how to change order of photos in an album?

You can only change the order of the photos in an album you create yourself, not in the albums that are automatically created by Photos for you. The predefined albums (Recents, Imports, Media Types, etc), are having a fixed sort order, specific for the particular album.

• For all albums you create yourself (smart albums and standard albums, shared albums) you can sort the photos automatically using the menu item "View > Sort" while viewing the album.
• For example:

• For standard albums you also rearrange the photos manually by dragging them as described by Yer_Man.

Drag and drop. Note however, that you cannot do this in Smart albums.