Everything is under Account & plan, then Your plan, which shows your plan, its price, when it renews and what you have used this month. Only the owner can change it.
To upgrade, press Upgrade, choose Pro monthly or yearly and go straight to checkout. To change the card, switch between monthly and yearly, or cancel, press Manage or cancel plan, which opens Stripe's billing page. An upgrade is charged straight away.
Pro is 9.99 GBP a month for each person, up to three, and a flat 39.99 GBP from four people on, so the bill follows your team up to that cap. When the price changes because someone joins or leaves, the difference for the rest of the period is charged or credited at once, worked out to the day. A fifth person changes nothing, because the price has already stopped at 39.99 GBP.
Cancelling does not stop Pro straight away. Your plan then says it is set to cancel on a date, and you keep full access until then. After that you are back on Free's limits. Nothing is deleted: the booking page gallery and links are hidden until you come back, people over the free allowance are parked, and gift vouchers you have already sold stay good.
If you bought Pro in the FullDiary app, it is changed or cancelled where you bought it, in the App Store or Google Play, and the app's Pro screen changes how many people it covers. A plan bought on the website is changed on the website, and works in the app too.