Updating version of existing file.

There is a dropdown having all the names of files stored in knowledge center. User will select the desired file and click on update button. After clicking on the button, the desired file should be updated. I have saved the selected file in a new process variable of type document in script task and then given the same variable as target output in user input task having the value of the new file upload variable. Its giving an error "Invalid Destination Folder".Please help.

OriginalPostID-237323