If the stain is from dirt and water, you should wash your swing flag to remove the dirt. If the stain is only from water, either: (1) place a damp cloth or paper towel over the water stain and dry iron the cloth or paper towel leaving some moisture in the fabric to air dry; or (2) hold the stained area of the flag over a source of water steam for 30s (be careful not to burn yourself), leave to air dry and repeat until the water stain is gone.