Tamim, you can try this help page for how to change your Adobe plan or subscription online:
https://helpx.adobe.com/manage-account/using/change-plan.html
If that works and you can upgrade your plan online, then note the following:
"What happens after you change your plan?
After you change your plan:
- The existing plan is canceled and you get a refund for it.
- The new plan is effective immediately and you will receive a charge for the new plan.
- Your monthly billing date changes to the date when you change the plan.
- For yearly plans, the new renewal date is one year after the date on which you change the plan. For example, if you change your plan on June 14, 2019, the new plan will renew on June 14, 2020."
As to whether the 14-day refund period would still apply on a plan upgrade, that's a little hazy. The reason is because Adobe's Subscription and Cancellation Terms say "within 14 days of your initial order"... (emphasis added) It's unclear whether that would refer back to the first/original plan you have, which is your current Acrobat subscription.
https://www.adobe.com/legal/subscription-terms.html
Hope that helps explain a bit further.