The brush heads are meant to fit snugly into the handle and not come out during brushing. Once inserted, there shouldn't be a gap between the brush head and handle. To remove a used brush head, pull hard on the brush head directly away from the handle. Insert your new brush head by pushing it into the handle until it is tight.  We guarantee Grin and Nada toothbrushes – so if you continue to have problems, please contact us at hello@trynada.com