1. Start by selecting the cell where you want to display the sum.
2. Type the following formula: =SUM(OFFSET(reference,0,0,-n,1))
   - "reference" is the cell from which you want to start summing.
   - "n" is the number of cells you want to include in the sum. This value can be inputted by the user.
3. Press Enter to calculate the sum.

For example, if you want to sum the values in the 5 cells above the current cell, you would use the following formula: =SUM(OFFSET(A1,-5,0,5,1))

This formula will sum the values in cells A1:A5.

Note that the OFFSET function returns a range of cells, which is then used as the argument for the SUM function. The first two arguments of the OFFSET function specify the starting cell (reference) and the number of rows and columns to offset (0,0). The third and fourth arguments (-n and 1) specify the height and width of the range to return.