TL;DR
Use the profile menu to switch school years. An old bookmark can keep opening last year's planner even when a new year is available.
Choose a year
Open your profile menu.
Check the school and year of the active profile.
Choose the current-year profile, or open the list of other profiles to find a previous year.
Confirm the year after the planner opens.
For a fresh starting point, open studyo.app and replace an outdated bookmark after selecting the correct profile. Do not create another account to change years.
A previous year must still be available to your account. Schools control when new years are prepared and activated. If the expected profile is absent, check your email identity, then contact the school.
Teachers can import tasks and notes from a previous year; switching profiles alone does not copy them.