Choose Home tab→Select group→Select 
.
In the graphics window or PathFinder, select one or more features or faces to mirror and copy.
Choose Home tab→Pattern→Mirror 
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Define the plane about which you want to mirror the features. You can select a reference plane or a planar face as the mirror plane.
Tip:
Features, faces, face sets, and the design body are valid elements to mirror.
You can use the Persists Mirror Associativity option to control associativity between the feature being mirrored and the mirrored copy.