Use the Intersect Surface command 
to mutually extend or trim surfaces to a common intersection. You can extend surfaces up to other surface bodies. The trim capabilities support trimming multiple bodies. The intersect command can select only surface bodies, and all surface bodies act as both a target and tool.
Note:
When selecting surface bodies, selecting only two bodies enables both Extend and Trim operations in the next step of the command. Selecting more than two bodies enables only the Trim operation.
The example below shows a surface being extended.
The example below shows a surface being trimmed.
The example below shows a surface being trimmed via the region selection.
Extending and Trimming a surface