For most types of damages e.g. worn down wheels, when some part of the wheel is lost or if the wheel is flat on one side the only option is to replace the entire wheel.
In cases where the wheel does not longer run smoothly but is otherwise intact, we recommend checking the wheels for debris, hair or other objects being stuck within the wheel. After cleaning the wheel you can try lubricating the wheel with a silicon spray or similar lubricant to make it roll smoothly again.
Warning: Please make sure no lubricant gets on the suitcase or to the inside of the suitcase as it could leave stains on your clothing.