So, from the Fire TV home screen, navigate to Settings and scroll down to Applications. From there, select Manage Installed Applications and find Disney Plus in the list. It's like, right there, waiting for you to update it!
Once you've found Disney Plus, select it and choose Update if an update is available. It's pretty straightforward, and you'll be updated in no time. You might need to restart your Fire TV, but that's no big deal, right?