You can change your active Dauber by visiting your Profile. From the top-left corner of the Bingo lobby, tap on your photo/ silhouette image to open your Profile. From there, tap on the tab that says "Daubers" to view a list of Daubers that you own. Tap the blue Activate button to select the Dauber you want to use for your Bingo rounds.

To purchase more Daubers, select the "Dauber Store" tab from your Profile.