Before you set different prices depending on the season, make sure you have read this article on how to set up your seasons.
When adding accommodation or package inventory you will come to the pricing tab, and there is a drop down which lets you select the pricing as "Year Round" or "Per Season". Select the appropriate setting.
You will now see "per season" pricing options when you are in the Pricing tab of the respective accommodation.
Select Add a Price, fill out the required information and select which season this corresponds to.
To add more pricing for a different season, click on "add a price" and go through the process again.