Setting a new Oshi-men:
Log in to Account Settings.
Select "Oshi-men" from the profile settings.
Choose your Oshi-men and press "Save".
Changing your Oshi-men:
Go to Account Settings and select "Edit Oshi-men".
Choose a new Oshi-men.
Confirm the change.
※ If you change to a new Oshi-men, the streak for that member will start from zero. However, if you switch back to a previously selected Oshi-men, the previous streak will be retained.