Hi there, @mateen.

I'm here to guide you throughout the steps so that you can successfully add the customer type to your customers in QuickBooks Online. Please be guided by the steps below:

1. Go to the Sales tab and select Customers.
2. Choose the customer on the list you wish to modify the customer type by clicking their name. (I added this screenshot for Steps 1 and 2.)
   
   
   
   
3. In the customer's profile, click the Edit button located beside the New transaction button.
   
   
   
   
4. Select the Additional Info tab then, click the drop-down icon under Customer Type.
5. Verify the details and click Save once done. (Also, please check below these screenshot for Steps 4 and 5.)
   
   

I can see that this article can be handy in the future about managing your customers' transactions: Send online invoices in QuickBooks Online.

You can always get back to this thread if you need anything else. I'll be here to help you. Keep safe.