Sergey,
I discovered that my proposed inclusion of WHERE in the update string as well as yours are working but you still have to include the macrosubstitution for the lprFieldValue. In your expression it becomes:
lcUpdateString = [UPDATE BEST SET ] + lprFieldName + [ = '] + [&lprFieldValue] + ['] + ;
[ WHERE BESTNR=] + Transform(nBestnr)

However, it works now and I'm very greatful. Thanks.