Choose Surfacing tab→Surfaces group→Trim Surface 
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Select a surface that you want to trim.
Select the input elements that define the boundary to which you want to trim, then click the Accept button.
Position the cursor on the side of the surface you want to remove and click.
Finish the feature.
Tip:
You can use a curve, reference plane, or another surface as the input elements.
If the input element you are using to define the trim boundary does not extend to the edges of the surface you want to trim, the trim boundary element is extended linearly and tangent to the input element.