To unmaster a target:
Navigate to the desired activity graph with has a target you want to unmaster.
Click the name of the target at the bottom of the graph.
Hover or click on the mastered phase line to open a pop-up modal with Edit and Delete options.
Select Delete.
On the "Remove Target Mastery" modal, select a new phase for the target from the drop-down.
Click Remove Mastery.
The target is no longer mastered.