How can I have vat registration no on my invoices instead of mauritius revenue authority registration no? I have changed the name of the tax agency to vat but no effect

You can add the VAT registration number in an invoice's header, rezah-c.  

You can go to the Custom form styles panel to customise an invoice and add the VAT registration number. 

Here's how:

1. Click the Gear icon on the top menu.
2. Choose Custom Form Styles.
3. Tap Edit on the invoice.
4. Select Content, and tap on the bottom half of your invoice on the right hand side of the screen. Here, it will allow you to add additional details into the box and a footer.
5. Press Save. 

For more details about customising invoices, estimates and sales receipts in QuickBooks Online (QBO): 

• Customise invoices, estimates, and sales receipts in QuickBooks Online 
• Add, customise, or remove logos on sales forms 
• Import custom form styles for invoices or estimates 

Keep me posted if you have any other questions or clarifications. I'm ready to help. Have a nice day!