Hello Bob,
Thanks for reaching out to the Apple Support Communities. We understand you'd like information on how you can extend your AppleCare+ coverage for your iPhone SE.
If you paid in full upfront for AppleCare+ coverage you'll be eligible to extend it monthly after the initial contract period is over, you won't see the option to extend before that time. Check out the following information from this article: Extend your AppleCare+ coverage.
"In Australia, Canada, Germany, the United Kingdom, and the United States, you can purchase the new coverage within 60 days of the end date of your original coverage. In Japan, you can purchase the new coverage within 30 days of the end date of your original coverage."
You can find the step to follow to check when your coverage ends by following the information under the "How to check when your coverage expires" section of the article.
All the best.