Please tell How to open Activation lock for Apple Watch.

If the watch is activation locked to another person, you must ask them to remove the lock. Activation lock is one of the best theft deterrent systems out there, there is no way that you could avoid that.

If the watch is locked to an old Apple ID of yourself and you cannot recover the login information of that apple id, you can start an activation lock support request at Apple

See this link for further reference

How to remove Activation Lock - Apple Support