To change the date format in online Excel, you have to:
- Select the cells with dates
- Click on selected cells with right mouse button to show the menu, choose Number format
- In Date section, select of available date formats
- If you need to use other than available date formats, change Location (there are different date formats in US, Europe etc.)
Online Excel is a free product with limited functionalities. Creating custom number formats isn't currently supported in the web version of Excel.