Change is signed by making both your index fingers into a hook while the rest of your hands are made into fists. Touch your hands at the wrists, then reverse your hands (changing their position).
An ASL alternative to change is to simply form both hands into fists (or the 'S' letter) and change their position at the wrists.
Change is used by toddlers to indicate when they need their diaper changed. Learning to communicate when they need to change their diaper helps to develop their communication and help develop their awareness, which will be useful when they start potty training.
Starting to develop communication about when a diaper needs changing is a first step toward potty training. As your baby starts to be able to communicate before they need to go to the bathroom, they will become ready to be potty-trained.