Hi @Maryem4
One way is to use professional tools (turned on by going to account and settings), and create a rule set with a discount that you can apply to dates of your choice.
Or, on your calendar, click the availability tab. Under discounts, scroll to "Set up discounts". There's an option to set up trip length discounts (in line with what you are looking for).
Alternatively, you can keep all nights priced equally and add a cleaning fee. This adds a "per booking" charge to your fee structure, meaning shorter stays are relatively more expensive. It's a good way to match income and expenses, as guest changeover days cause significantly more costs than non-changeover days.