If you've reached your limit for offine devices with Spotify, or having trouble getting a new device registered, simply go here, log in and remove the device(s) you want to.
Important: Doing this this will result in your offline data being removed across your offline devices. You will need to download your offline music again.
When syncing offline content, always be beware of possible data charges from your mobile network provider.
There's also some info here, under the section "Manage your offline devices" you can check out.