Replace

exec ('use ' + @DestDB + ';UPDATE PS_PV_SRCH_RUN_CTL SET FILE_DIRECTORY = ''\\serverxxx\psoft\verity\''' + @DestDB + '''')

with

print 'use ' + @DestDB + ';UPDATE PS_PV_SRCH_RUN_CTL SET FILE_DIRECTORY = ''\\serverxxx\psoft\verity\''' + @DestDB + ''''

and you'll see what's wrong with the syntax of your query.