At the left side in Menu Bar, there is option of button "Setting". You can find setting option in "My account" as well. We have to click on it.
After clicking on the setting, we will go in the option "Invoice". here we will click on the "PDF Header".
Here we can write which we want to mention on the top of the invoice page. Like : Invoice, GST Invoice, GST tax invoice or Tax invoice etc.
After that we will click on the button "Update".