Well, it depends on how you did it. If you changed it by making the SD card "Internal" instead of "External" storage, then you will need to undo that. To undo it go into "Settings" and then "Storage".
IMPORTANT NOTE: If you set it up as "Internal" storage and want to change it back, you will need to move all data off the card first and move apps to the true "Internal" storage or lose them.
Now if you only set it up for a specific app (say for music or browser, etc.,), then you would need to go into that apps "Settings" and change the location.