Yes, it actualy not in documentation. Please we are not your slave and I feel some arogancy from your questions and reactions so don't just throw simple question and take your time to describe what you want, try to find out if there is no solution on forum or in manual(this is no the case).
So I did reverse ingeneering from Seahub and find out that is posible by http PUT request.
curl -X PUT -d "old_password=<old-library-password>&new_password=<new-library-password>" -H 'Authorization: Token 0485aefe45448afcb3b5f7c630d19f1a41179b09' https://cloud.seafile.com/api/v2.1/repos/<repo-id>/set-password/
@daniel.pan May this should be in DOC. I did some PR on WebApi documentation but it's not still merged so I'm not going to do it cause waste of time for me.