So you want to make the SD card "External" storage again?
Well, if you want to keep anything on it (files) you will have to move them from the card to the phone. The same goes for any apps you had on the phone and then moved to the card. They will have to be moved back to the phone. However, any apps you downloaded and installed directly to the card, after it was made internal, will now be lost. They cannot be moved to the phone, but will have to be redownloaded directly to the phone once the card has been converted back to "External" storage.
The steps to change it are the same as to make it "Internal" you just choose "External" instead this time. You can also just format the card and then restart the phone. It should allow for it...