How do i swap icloud devices when upgrading iphone?

i have just upgraded from iphone 4s to 5s, i have succesfully brought all my info from icloud to my new phone which is all fine.

when i come to back up the new phone to icloud it is doing it is a seperate device to my old phone therefore doubling the storage space requirement, i don't want this to happen as i will be selling the old phone.

i dont want to buy more storage because i clearly dont need it

i want to remove the old phone from icloud and replace my new device on that account

how do i do this?

thanks