Hey there, Rosalee. Thanks for stopping by with your question.
It should be pretty easy to update your daughter's age. First, log in to your
MyDisneyExperience account. Then on the MyDisneyExperience drop-down menu, you should see the "My Family & Friends List" option and you'll want to select that. From there, you'll scroll until you see the name of the child for who you would like to update information and select "Update". Once there you can update things like their name, age and birthday. You can even change her character icon to match what she loves now! I know my daughter likes to switch hers out and there are so many cute ones to choose from right now. When you are done you'll hit "Save" and you are good to go.
MagicBands are actually a one size fits most, Rosalee. To make the MagicBand smaller for kids, you are actually going to peel the outer ring of the MagicBand off. This leaves behind a smaller band that fits on children a little bit better. But you don't need to worry about updating your daughter's age for the MagicBand, itself, however, it is a good idea to get her information up-to-date.
If you find yourself having trouble updating your daughter's birthday, you can always call (407) 939-1936 and a Cast Member will be happy to assist you. They'll be able to enter your account and manually change the information that way as well.
I hope this helps. If you have other questions, please feel free to come back and
ask me. I'm here to help.
Have fun and enjoy!