Change Y Axis on graph

Hi All, in the attached workbook I have some date and a graph. The vertical axis shows 0-25 upwards but what I want is that to show 1 at the top down to 24 at the bottom or better still 1st to 24th. I've been looking at videos on utube to try and work it out how to do it but when I click on the Y axis as instructed I do not get the option "Format Axis" which it shows. Many thanks.

Note: The graph shows where a team was positioned in the league after each game played, hence why I want 1st at the top.