code: func (c *Collection) Update(selector interface{}, update interface{}) error {
  	lerr, err := c.writeQuery(&updateOp{c.FullName, selector, update, 0})
  	if err == nil && lerr != nil && !lerr.UpdatedExisting {
  		return ErrNotFound
  	}
  	return err
  }
docs: // Update finds a single document matching the provided selector document
  // and modifies it according to the update document.
  // If the session is in safe mode (see SetSafe) a ErrNotFound error is
  // returned if a document isn't found, or a value of type *LastError
  // when some other error is detected.
  //
  // Relevant documentation:
  //
  //     http://www.mongodb.org/display/DOCS/Updating
  //     http://www.mongodb.org/display/DOCS/Atomic+Operations
  //