Visa extension services are available only in Kathmandu and Pokhara. Every visa extension requires your passport, the fee, one photo and an application form that must be completed online first. Collect all the necessary documents before joining the queue in the office. Visa extensions are available the same day normally within two hours.

You can get up-to-date visa information at the website of the Department of Immigration:
http://www.nepalimmigration.gov.np/