99% sure they simply slide on. However, it's likely the metal is slightly barbed to prevent them from just wiggling off.
Provided no locking mechanism is visible (or feel-able) anywhere around the handle part that the stem goes into (like a set screw or a locking tab that latches onto the metal stem), just pull straight with even force around the handle, and give it a slight wiggle up and down as you pull. They should just pop off.