The sub domain for a collection controls the URL for all OnePagers inside of it.
To change the sub domain just go the Collections page inside of your account. Click on the collection you want to edit. You can find the Settings tab which will show the input to change the sub domain.
All OnePagers in the collection will automatically update the URL.
If you change the sub domain the current OnePagers using that old sub domain will still work at the old URL.