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Before deleting a Master, check your Apps to remove any references to that Master.
When you are ready to delete, go to the Master Tab, and find the Master in the left menu.
Click on the delete icon.
When you click delete, KiSSFLOW will automatically check if there are any Apps that use this Master you have missed. If it finds any, it will tell you the name of the App and the name of the Field that references the Master.
After removing the fields, remember to publish the App again.