That depends on your status.

• If you are a permanent resident, you can come and go from the U.S. at any time.
• If you are applying for legal immigration status, you should not (in most cases) leave the U.S. until your application is complete.
• If you are in a removal proceeding, you cannot leave the U.S. until your case is decided.
• If you are an undocumented immigrant and you leave the U.S., you may not be able to return to the U.S.