Hello Gillian-murphyen, thanks for posting on the Community page, To change the sales price/rate of a product item, follow these steps to proceed:
1. Go to Sales and select Products & services.
2. Find the product or service you want to change.
3. Tap Edit in the Action column. Change the Sales price/rate.
4. Once done, click Save and close.
For more details about this process, refer to this article: Add product and service items to QuickBooks Online.