Couple of ways to do it @Lara177,
1/ go to your listing details, click pricing and click on length of stay discounts, at the bottom it should say "You can also set a custom price for a specific week or month." click it and have a play. ps sometimes it doesn't appear, refresh your screen and it should pop up.
2/ go to your calender and with the left mouse button, highlight the days you want to change the price. On the right side of your screen will appear a list with "dates selected" "availability" and "nightly price". Alter the nightly price to whatever you want and click save.