How to use Rest API PUT Call Properly

We are trying to update one credential attribute from credential vault using REST API PUT process but after running it , it is giving us the below error - 

{"code":"optimistic.lock.exception","correctiveAction":"To continue, please try again.","details":"Unable to save your changes because this object may have been edited by someone else.","message":"Unable to save changes"}

Can anyone suggest what should we do in order to save the changes, or how can we update variable using another API call ?