I have items in one album that I now would like to have in another album

iOS

Open the album where the items are now
Tap the 'select' icons in the upper right corner (box with check mark)
Tap the items you would like to move, so that they are highlighted and have a check mark
Tap the move icon (first icon from left, box with arrow to the right)
A list of all your albums appears
Tap on the album in which you would like the items to be moved
You will receive a confirmation of the number of items that have moved in the chosen album

Android

Open the album where the items are now
Tap the 3 dots in the upper right corner
Tap ‘select items’
Tap the items you like to move so that they are highlighted and have a check mark
Tap on the move icon (third icon from the left, box with arrow to the right)
A list of all your albums appears
Tap on the album in which you like the items to be moved