Question: how to change date format in excel to dd/mm/yyyy by default?
Answer: Select the cell (or cells) you want to format.
Choose Cells from the Format menu. ... 
Make sure the Number tab is selected.
Click Date at the left side of the dialog box. ... 
Using the Locale drop-down list, choose a country or region that uses the date format you want to use. ... 
Select the desired date format.
Click on OK.