Even if a column is hidden you can still get its cells and edit their value. For example, if you have hidden the second column "B" you can get the cells in that column and set a different value using the SetValue() method of the CellSelection object.
var workSheet =
ColumnSelection column = workSheet.Columns[1,1];
CellSelection selection = workSheet.Cells[1, 1];
"Your custom Text"
More information about setting different properties of the cells is available in the Get, Set and Clear Cell Properties
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