HP QC - excel import - how to update field value ?

I am importing requirements and test cases in requirement and test plan module in hp qc 10, what are the steps to update values for already imported data?

e.g. I want to update a list of value  from "2-high" to 2-very high" for all the 100 existing records in QC. After import,  20 out of 100 records will return a error stating "duplicating requirements...". The error message, however, is incorrectly reflected because all 100 records are existing, not just 20. As a result, only 80 records gets the value updated.

Please help.

Thank you.

Icy