While inside your Squad that you're admin of, tap on the hamburger menu at the top left of the screen.
Tap on the 3 dots icon at the top of the screen that just opened up.
Tap on the setting cog next to the squad you'd like to manage, in this case "Your Squad Name".
You will then be on the 'Squad Info' page, from there, scroll down, and tap on 'Upgrade Plan'.
Finally, decide between a monthly or yearly subscription and scroll to view the different plans, and tap on 'Subscribe' once you've decided. You will then follow the prompts for the app store billing page to pay.