When you want to upgrade your subscription, you need to go to the license tab and select the subscription level you want and make a purchase at the new subscription level. The time you have remaining on your subscription will be prorated and added to the subscription you purchased. The license tab will tell you when your account will expire so you can see your prorated expiration date.
For example, if you upgrade from professional to elite, your time remaining on your account will be reduced by about 1/3 (because the difference in price is about 1/3). So lets use this concrete example:
It is January 1st, 2020. Your professional account expires on April 1st, 2020. You are upgrading to elite and purchasing an elite subscription. Your current subscription will be prorated to March 1st, 2020, and with the year you purchase, your new expiration date will be March 1st, 2021.