You absolutely do not have to pay for any upgrade to our products. There can be several reasons you are not seeing a Download button instead of a Buy button.
It may be that you are not logged into the app store on your iPhone or iPad. If you are not signed into the store there is no way it can tell that you already own the app.
It may be that you are signed into the store but with a different account. The only way you can get a free update is if you are updating the app on the same account that you bought it with.
We find that customers often forget what account they used to originally buy an app. Or they have several iPhone or iPad accounts. If you are not sure which account you used to buy our product look for the email receipt that Apple sends whenever you buy an app. The email address will be the account you would sign in with.
You will know you won’t be charged for an app when instead of seeing a price on the Buy button it says Download.