Choose Features tab→Pattern group→Mirror Assembly Feature 
.
Select one or more assembly features to copy, then click the Accept (check mark) button on the command bar.
Define the plane about which you want to mirror the features.
Select the parts that you want the feature to modify.
Finish the feature.
Tip:
You can only select assembly features. You cannot select assembly-driven part features.
The Select Features Step allows you to specify which features you want to mirror and copy. When you set the Feature option, you can select assembly features which remove material, such as cutouts and holes. When you set the Body option, you can select assembly features which add material, such as a fillet weld feature, or a protrusion feature. You can only create assembly features that add material when you have specified that the assembly is a weldment assembly.
The Select Parts Step allows you to specify which parts you want the new feature to modify.