Edit a checkbox to completed

Edit a checkbox to completed

I am having the following issue and can't seem to find the answer. 

I have 2 forms in my application. 1 creates a request and the second for is used for follow-up.

On the second form I created a link that pulls a drop down list of the requested with completed == false from the first form.

What I wanted to do is on a succesful submit the field of the selected record ID the Completed field becomes true.

Any ideas on how to do this?

Thanks in advance for the assitance!