U2713HM, stand problem

Hi

I have recently purchased a used Dell U2713HM monitor and would like the screen higher but when i raise the monitor to the position i desire it just slides back down to the bottom of the stand under the weight of the monitor.

I can't find any button etc to lock it in place so i guess it is just supposed to stay in the position which it is placed.

I just wondered if that is the case is there any way i can adjust the stand to hold the weight of the monitor or do i need to get a new stand.

Any help would be appreciated.              

 Thanks Mike