Go to the My calendars page. Click Edit on the calendar which you want to extend.

Before adjusting the time period, please see Why does my calendar only show 6 months (or similar) into the future? for important information.
Go to the My calendars page. Click Edit on the calendar which you want to extend.
Open the Time period tab and adjust the time periods to your liking.
The Time period tab is not shown on phones due to their small screen size. You'll need to adjust this setting with a tablet or a computer. The extended calendar, however, will work fine with phones as well.
Finally, click Save.
If you need more than two years (for some calendars, the limit is shorter), a calendar subscription will not work. It is a wrong tool for this problem.
Instead, you may download a calendar (.ics) file which contains events up to 5 years into the future. This file can then be imported into your calendar application.
When downloading a calendar this way, you should understand that
To download a single file with up to 5 years of events into the future,
Updated Wed, 08 Feb 2023 12:08:23 +0000 UTC