Open the contract and go to Price rise. Pick how the price changes — a percentage, a fixed amount, or a new value — the date it starts, and a one-line reason. The table shows every item's price now and from that date: the contract's annual value and, if you leave the tick on, the per-visit price on every round under it. Tap Apply price rise.
What happens. From the effective date the contract value and the rounds' prices change (immediately if the date is today or past, otherwise on the day). Work done before the date still bills at the old price even if it's invoiced later, because each visit is priced by the price in force on its date. The contract keeps a price history: date, old and new, who and why. A pending rise can be cancelled before it takes effect; an applied one is undone by recording another change, so nothing is lost.
Telling the customer. Leave "Email the customer a notice" ticked and their billing contacts get a contractor-branded note listing each item, old and new, from the date, with your reason. You can re-send it from the history.
Several contracts at once. On the Contracts list, open Bulk price rise, untick any you want to leave out, and apply the same percentage to the rest — an annual review in one go.
A single round that isn't on a contract has its own Price rise section on the round page. For end-of-term repricing based on what the contract actually cost you, use the Renewal tab instead.