Excel sheet columns updated to null but...?

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historians

hi,
I have an application where I have to update an excel sheet with each column set to null.But when it gets updated to null,the rows are getting shifted to rows which do not start from the first row.I mean,excel sheet is created with gaps in first few rows and then my data is inserted there.I want my data to be inserted starting from 1st row.
I have to enter the tasks which are non repetitive only...so I update each column to null before inserting the new tasks so that only the non repetitive tasks are inserted into the sheet.But when i update the columns,repetitve tasks are set to null...but the non repeitve tasks are not inserted in the first row.first few rows are left blank.I want to avoid thiese blank rows.plz help