I have documented the requirements in QC. But i would like to export into excel and when required again import into QC. So i would like to know the procedure for exporting the requirements into excel from QC.
Unless you're using QC9, there is no standard way to export Requirements from QC to Excel. In QC9, you get a grid view, similar to the Test Grid view of the Test Plan module, that would allow you such an export.
You may want to try the following detour : generate a report, customize the contents of that report and make sure that you select "Grid view" ; then, in the report window, select the text, copy it to the clipboard and paste it into Excel.
The obvious problem will be with the memo fields, such as the requirement description, which will appear on new Excel lines instead of being in an Excel column, so, you'll need to rework the resulting Excel worksheet.
Another way to do it is to program your own Excel macro, using the OTA object model to get the data out of QC.