You will find the option to 'Become PRO' in the Settings tab (click here to open Settings in Delta). Tap "Learn More" to open the screen shown below. It will show you the major PRO features (such as unlimited exchange account connections, portfolio analytics, real-time prices, the ability to sync 5 devices, monochrome mode, etc.) and the annual subscription fee in your local currency. You can learn more about what comes with PRO here.

Thank you for considering PRO!

PRO TIP: If you have an issue with buying PRO, check out this guide here.
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​PRO TIP 2: If you are looking to find out how much Delta PRO costs, you can learn more about this here.