How can I increase all sales items with 10% ?

Nice to have you in the Community space, Lynda.

You'll have to edit and manually adjust the price of each items in your QuickBooks Online (QBO) account. I'll show you how.

1. Go to the Sales menu and select the Product and services tab.
2. Hover to the item you want to edit the price and click Edit under the ACTION column
3. Scroll down to the page until you'll see the Sales price/rate field, and update it. You can utilize the internet to get the calculation of how much amount you'd adjust with the 10% increase. 
4. Then click Save and close.

You can also check the Edit a product or service item section in this article for more details: Add product and service items to QuickBooks Online.

Another way is to export your items to MS Excel and adjust the price of multiple items from there. Once done, you can import it back to your account. You can utilize this link for the complete process: Import products and services into QuickBooks Online.

I can see having the option to adjust the sales price of all your items at once is beneficial to your business. I recommend sending your feedback to your Development team. This way, they can review your suggestion and might consider adding it in the future. 

You can also utilize a third-party tool that lets you update the price of your items at once. Just hover to the Apps menu and look for an app from there.

Furthermore, I'm adding this resource that you can read for your reference in seamlessly tracking your products: Set up and track your stock in QuickBooks Online.

I'm all ears whenever you need help managing your products in QBO. Have a great day ahead!