Update field in one document based on a field in another document

Hi all, I am pretty new to notes. I have a button on a form that approves a request. When this approval is given I would like to increment a field on another document in the same db. I would like to do this using @commands. Does anyone know how this can be done. A code snippet would be great thanks.

Subject: update field in one document based on a field in another document

@SetDocField( documentUNID ; fieldName ; newValue )

Use in a button, a form event such as QuerySave, or a computed field.