To upgrade a certain account you're managing in ManyChat to the Pro plan, you need to click the blue 'Activate Trial' button on the left panel.
You will be presented with this window then:
Click 'Start Pro For Free', then provide your billing credentials, and hit the 'Pay' button here:
Note that to set a Trial, you will need to enter your credit card data. Also note that once the Trial is over, you will automatically get charged for the following month of Pro subscription!
That's it — the page should become Pro right away!
PS: If anything goes wrong, please check out our article about Billing issues.