Castors or glides can easily be changed over as follows:

Turn the chair upside down and support the weight on a table or desk which is stable.

Pull out the castor or glide by holding the main wheel and casing.   The metal pin which fits into the chair base should pull out of the chair base and remain with the castor.   If this is not the case, the pin can be removed separately with a pair of pliers or mole wrench.

Insert the required castor or glide into the foot of the chair base.

Ensure that all five castors/glides are inserted correctly and evenly before returning the chair to it's upright position and sitting on it.