I'm unable to find the correct syntax to execute a PUT command to update an existing Test Case work item in           
  which I added a requirement link.          

  Followed the suggestion mentioned here                                                                         
   1. GET the TestCase resource from the first project.                 
                                                                        
    2. Extract the oslc_qm:validatesRequirement link(s).                
                                                                        
    3. GET the TestCase resource from the second project.               
                                                                        
    4. Add the oslc_qm:validatesRequirement link(s).                    
                                                                        
    5. PUT the TestCase resource to the second project.                 
                                                                        
Unfortunately I haven't managed yet to find the exact commands for the PUT command in POSTER ( step 5. ). Appreciate some quick inputs here.

Thanks