If you have not chosen for 'automatic renewal' your subscription will end automatically.
Otherwise you can cancel your subscription at any time you like. Go to your Profile and select 'Edit account' under account. Turn off 'automatic renewal'.
If you cancel your subscription and the amount of albums supersedes the maximum of 5 for a free account there is no need to worry. This will not affect your existing albums, you will just not be able to upload any new content. Upgrade again to create new albums.