A title can only belong to a single Publication Season. Unlike Subjects and Themes these can only be added from the individual Title window to do this you need to follow the following steps.
Navigate to Title Manager/Books in the left hand menu.
Select the title that you wish to edit, and click on it name.
Select the Publication tab.
You will see that there is a section to add in the Publication Season as per image below.
The list will populate from the publication seasons that you have created See Setting Up Categorisations
Once you have selected the correct season you click on Update and this will be saved.