To edit the Predefined Times for each of your social accounts, open Your Account on the side menu where your profile picture is, and go to the Social Accounts section.
There, find the social account you want to set up and click on the Settings button (gear icon):
Then, a pop-up window will open and you will find the Predefined Times tab.
Finally, add the predefined times you want for each day of the week. You can also speed this up by selecting Every Day, Weekdays, or Weekends. This will make it so that each predefined time you add is automatically set up on those options.
You will be able to schedule posts in your Predefined Publishing Times from the Composer and from the Bulk Uploader as well!