To remove a brush head that's stuck on your toothbrush, try running the toothbrush’s top section under warm water for at least ten seconds. Then, see if you can pull it off. You might have to give it quite a forceful tug, but it should come right off.
Sometimes toothpaste can build up under the brush head and solidify to quite a tight bond. It can even cause part of the inside of the brush head to break off when trying to remove it, leaving a piece behind on the metal shaft. To prevent this, we recommend the cleaning and maintenance routine noted in the manual: 1. After each brushing session, remove the brush head and thoroughly rinse both the brush head and the handle under running water. 2. Wipe the brush head and handle dry with a towel or cloth before putting the brush head back on the handle and storing the toothbrush for the next use.
This should prevent any toothpaste buildup.
Once you remove the brush head, check to see if there's any material stuck to the metal shaft. If so, follow the same process of running warm water over it for at least 10 seconds then pulling it off.