You can upload your own letterhead as a PDF file in the settings.
You can easily create your own letterhead in Word (or a graphics editor). Just drag your logo into an empty DIN A4 document into your preferred location and add a footer if required and save or export the file as a PDF.
The file can be uploaded to sevDesk later and set as a default via the star. This way the letterhead will be automatically added to any future document.
You can also change the letterhead before you send an invoice in the preview via the following menu: