Processes in Microsoft 365 for setting up Office apps, redeeming product keys, and activating licenses.
You can change your plan via your Microsoft account on the website account.microsoft.com. Click Manage beneath your subscription.
Someone earlier in the thread said that the option to change to Office 365 Personal Classic only appears if you are close to the end of your current subscription. I had the option when I switched a couple of weeks ago and my current subscription completes in March.
Unfortunately there doesn't seem to be a way to change the plan immediately, so I still have to put up with the Copilot stuff until March...