How do I access iMessages from a year ago?

go to Settings > [your name] > iCloud and check the switch position for Messages

If the switch is ON, then you cannot recover your deleted messages (no need to read any further)

If the switch is OFF, then you might be able to recover your messages IF you have an iPhone backup that contains the messages you are trying to recover. In which case you'd need to restore your iPhone to that backup. In doing so, you'd likely lose some data between now and whenever your backup occurred.

Locate backups of your iPhone, iPad, and iPod touch - Apple Support

Restore your iPhone, iPad, or iPod touch from a backup - Apple Support

IMO, the odds that you have such a backup is pretty slim as you would have had to archive one.. or not have backed up for a year so that it's your most recent backup

In Settings > Messages > Message History - you have the option to choose 30 days, 1 year or Forever.

Chances are if you didn't choose Forever, the messages which are gone are gone. But you can change the setting now for future messages.