You will have to renew your passport, if you find that your passport is going to expire. To renew your passport, you need to file Form DS-82, Application for a US Passport by Mail. You may apply for passport renewal by mail and you need not go to a passport agency, to apply for renewal. Though it is possible to renew your passport in person, it is more easier to renew it by mail as your US passport will be mailed to you, after your application for renewal is approved.