First of all, here is an explanatory video on the subject:
To make a Quick Availability Change in the Schedule :
Click on Schedule in the Main Menu,
Click on the offer of your choice on the My Schedule page ,
Click on the Availabilities button.
Select the offer you wish to modify
Use the mouse to select the period concerned :
Day by day, by clicking on the given days
Week by week, by clicking on the week number
Month by month, by clicking on the cross to the left of each month
You can also combine specific days with several weeks and one or more months
Enter the new stock to be applied from the 'Remaining availability' box.
Then click 'Apply'.
Note: Just as you can make a quick change to availability, you can also make a quick change to prices.