I am trying to change the document type from "Tax Invoice" to "INV" in the invoice printout. How can I do this?

To change the document type from Tax Invoice to INV in the invoice printout, You need to follow the below steps:
First, confirm whether the invoice format is standard or designed.
If using standard format, follow the below steps:
Step 1: Go to Administration, then select Configuration.
Step 2: After Click on the Configuration option within the Administration section.
Step 3: You will find the invoice or document printing.
Step 4: In the Invoice Document Printing window, choose the Standard tab.
Step 5: Select the sales invoice. In the Sales Invoice window, find the field Invoice heading in which you need to select the option As mentioned below. After selecting this option, you have the option to change the document type from Tax Invoice to INV.
Step 6: By following these steps, you will change the document type from Tax Invoice to INV in the invoice printout.
or
If the designated format is used, then you want to include a customer's mobile number on a sales invoice, and you have a specific format to follow. Here's how to do it:
Step 1: Go to the Administration section.
Step 2: Then, click on Configuration.
Step 3: Within the Configuration menu, select Invoice or Document Printing.
Step 4: After that, go to the Advanced settings.
Step 5: Next, choose Sale Invoice.
Step 6: Now, select the particular sales invoice format that you need.
Step 7: To change the document type from Tax Invoice to INV in the invoice printout, you have to select the header option and go to the field  and enter a hard code like this:<^INV^,14,C>
Step 8: When you do this, then the document type will show the INV in the designated location, as per the format you've chosen.