How can I cancel my student plan and switch to individual?

When a Student subscription expires, it should automatically fall back to an Individual subscription.

Get an Apple Music Student subscription - Apple Support

Periodically, UNiDAYS re-validates your student status. If UNiDAYS finds that you're no longer a student or if you have a Student subscription for a full 48 months, your subscription automatically becomes an Apple Music Individual subscription.

Why not? What happens when you try to choose the Individual plan?

Have you followed these steps?: Manage your Apple Music subscription - Apple Support