Changing a price
Open Menu Library → Products, select the product, change the price, save. You can do the same from the POS under Menubook.
The new price applies to orders placed from that moment on. Orders already open keep the price they started with, so a table mid-meal isn't repriced under the customer.
Your POS picks it up within seconds. Your online store follows — how quickly depends on where you made the change, which is covered in Publishing your menu.
More than one price for the same item
You aren't limited to one price per product. Custom, bundle and alternative prices covers setting the price at the moment of sale, charging less when someone buys several, and running a separate price list for online orders or a delivery platform.
To change a lot of prices at once, use the pricing dashboard rather than opening products one by one.
Taxes
Your tax numbers are entered when your POS is first set up. Tax rates themselves aren't something you edit per order.
If your tax situation changes — a new tax status, a new kind of sale — contact support rather than working around it. What appears on a customer's receipt is regulated in Quebec.
There is one tax rule you do control, because it depends on how you sell the item: see Bakery items sold in sixes.