From your WordPress Dashboard, go to Appearance > Customize > Theme Options > Header > Header Button section.
Add your text in the Button Text field.
Insert the link in the Button Link field in order to redirect the button to the link.
Click on Publish.
Note: You need to have assigned Primary Menu to display the header button.
It is shown in the image below:
If you need another header button, it is available in Pro: Header Button Two Overview (Pro)
Published on: 23 / 07 / 2019