how do i erase my apple watch completely if it is lost i can not find it.

Are you sure you want to do that? It will allow anyone you finds your watch to then use or sell it. But if you insist, with a web browser, go to: https://www.iCloud.com and sign in with your Apple ID. Open Find My iPhone and select All Devices (at the top). Then click your Apple Watch, select 'Erase Apple Watch', and select Next until the device is erased. Finally click "X" next to remove your Apple Watch.