In the case of the purchase you should have got a email with the key. If did not make a note of the version you were running the date of the email might give you a clue.
If it was 2019 or 2020 then the upgrade to 2021 is free. If it was 2017 or earlier then you need a new key at a 50% discount. But to get a new key for 2021 there is a special process as 2021 is in preview/beta not released. Hence my questions to establish if I need to post that process for you.
Until 2021 is a released version, any keys from this link will not work.
Instead install 2021 and run Moneydance.
In Moneydance 2021:
If you are a new user that does not have a key click PURCHASE.
If you are an existing user whose key is valid for 2021 just carry on
If you are an existing user whose key was for 2017 or earlier and thus is not valid for 2021 click UPGRADE to purchase a new key at a 50% discount.
Note a 2021 key will not work with 2020 or earlier versions.
I think I was able to move back a week or so to a copy pre downloading some of the beta program and I am not getting the message of a demo copy. If this continues to be ok. will NOT download the beta copy. Probably will eventually need to pay them but can wait till I hear.
This program is not nearly as good as quicken was when it worked. Which it stopped working for me several years ago which is why I switched.