Use the buttons to select whether the values are to be preset or to iterate - then either click anywhere on the sheet and hit function button F9 or the "Calculate now" option on the "Formulae" menu item at the top. You can also select the preset values that start a new set of iterations.
Note - you will have to save this spreadsheet, as the upload will give you a "read-only" version. You will also have to make sure that you set "File/Options/Formulae/Enable Iterative Calculation". Sorry - Excel has default settings that are not helpful here!