Use the align tool in 3D view to align one or more elements with a selected element. See Aligning Elements.
Select the align tool, click on the element to use as a target (vertex, edge, face, surface, point, line, arc, reference plane, or level) and click again to pick the element to align with the target.
Before: 3D view of unaligned elements
Step 1: Click to select the element face to align with.
Step 2: Click to select the element face to align to it.
After: The elements are aligned.