How to Alter table( adding extra) columns using nodejs' orm module?

I am using nodejs orm module with mysql and i want to sync the models with the Database, when i am creating a model which is not in the db then it is working fine and the table has been created in DB but when i am trying to give existing Table's model with some column changes then db.sync() is not working. can anybody suggest me how to do that?

Thanks in advance :)