Hello @Maggie1,
The calendar layout has been changed recently and the help articles are still several revisions behind. It looks like Airbnb never bothers to update them.
One way to set a custom price for a specific period of time is as follows.
Go to your calendar and click on the first date for which you want to set a different price.
A box with "Start date" and "End date" will appear on the right. Enter the desired start and end dates and then enter the price you want in the "Price for the night" box. Then click the "Save changes" button.
Steve.