Before deleting a Master, check your Apps to remove any references to that Master.

When you are ready to delete, find the Master in the Master tab. You can use the search and sort feature to find it easily. 

You can delete the master directly from the card, or you can can also delete it after you open the master. 

When you click delete, KiSSFLOW will automatically check if there are any Apps that use this Master that you might 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 before trying to delete the Master.