Stop F@H (pause the currently running client first so that you don't loose the good workunit). Doing the following while F@H is running is probably a bad idea!
Locate the folder/directory with the data and log files for the F@H client. Within that directory you should see subdirectories such as "logs", "cores" and "work".
Within the "work" directory you will find further subdirectories for each workunit. The subdirectory with name "00" has the stuck workunit and the subdirectory with the name "02" has the good workunit (based on the posted picture, this may have changed if that workunit completed in the meantime and another one was loaded). The important part is that the value shown in the ID column in FAHControl for the work queue is also the name of the subdirectory under "work".
You can delete the entire "00" subdirectory as it will be recreated as needed by FAHClient. On restart (you may need to click Fold to un-pause) you will see log messages:
WARNING:WU00:Missing data files, dumping
Followed by connecting to the assignment server and downloading a new workunit.--
Got some spare cpu cycles ? Join Team Helix or Team Starfire!