I am not able to update database field because i dont know how to write groovey_script code for update...would some one please suggest how to write queries.....?

In my view, as long as working with any BPM products, regardless BonitaBPM or others, it is not recommended to operate on DB directly.

You can define BDM in BonitaBPM and any changes on BDM data will be updated into business database by platform.

If you want to operate non-BonitaBPM database, you can use database connector in Connector In or Connector Out of each step. Database connector can call your customized SQL statement.