To upgrade certain page you're managing in your ManyChat account to the Pro plan, you have to click blue 'Upgrade to Pro' button on the left panel:
You will be presented with this window then:
Click 'Upgrade To Pro', provide your billing credentials and hit 'Pay' button here:
That's it - the page should become Pro right away!
PS: If anything goes wrong, please check out 'Can't upgrade account to PRO' article.