You may set and switch between Stamps by selecting a Multiplayer quest from "Event",and clicking on the "Select Stamps" button located under the "Make Room" button.
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■ Steps
1. Select a Multiplayer quest from "Event".
2. Under the "Make Room" button, tap on "Select Stamps".
※ The Select Stamps button is to the right of the Back button.
3. Under "Favorite stamps" tap on the stamp you wish to change,
then under "All Stamps" tap on the stamp you would like to set.
4. Tap OK.
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※CAUTION: You are not able to edit Stamps in the middle of battle.