Welcome to JustAnswer
I am Pete, and I'll be happy to assist with your Excel question.
You should save the spreadsheet under a new name first, so that if any mistakes are made in clearing the old data, you will still have a copy of the original.
Then to clear all the old data, you will need to select any cells that have inputted data and press the Delete button.
If you have a lot of data to be deleted, there are a number of ways to select multiple cells at once.For example, if you want to delete the entire data from a column, you can click the column header letter, then press Delete.If you want to delete data from an entire row, you can click the row header number to select the row, then press Delete. You can also select a block of multiple cells by clicking and holding the left mouse button whilst hovering over the first cell, then drag the mouse to make your selection.