How to reset UsedRange on an Online Excel Worksheet

[reneegmol29]

I used the UsedRange all the time in Excel 2016 but I have a spreadsheet that is only online?  How do I reset the used line as when I hit CTRL+END it goes all the way down to Row 1048576 which is crazy.

Thanks for any help :)

[#2]

Are you not able to edit the workbook in Excel desktop?
Alternatively, deleting the extraneous rows should also fix the large used range problem.