I've set up a VAT, but I finally have to unset this and work without VAT. How do I come back and correct the initial setup about VAT ?

You've come to the right place, @pihemme.

You can edit your VAT setup in the VAT Center. I can guide you in doing so.

1. Go to the Taxes menu.
2. Choose the Edit VAT ▼ drop-down arrow.
3. Select Edit settings or Edit rates. 
4. Hit Save.

You can read through these articles for more detailed steps in managing VAT in QuickBooks: 

• How do I set up sales GST/VAT rates and use them on forms?

• Edit sales tax rate in QuickBooks Online

• Remove a VAT or GST rate

Visit us here again if you have other questions or concerns. I'm always here to help.