This should be nice and straightforward :)
On your iPhone/iPad
Just tap the Upgrade button at the bottom of your Database screen. You will see some options there, and you can choose whichever option suits you.
On your Mac
You can always upgrade by going to the Strongbox > Upgrade to Pro... menu item, or by clicking the '***Upgrade to Strongbox Pro...***' red button on the Lock Screen.
Thank you for your support,
The Strongbox Team.