Go to Admin → Settings and find the Branding section. Drag a logo onto the box, or click it to choose a file.
Convoro accepts JPEG, PNG, GIF and WebP up to 4 MB. Anything up to 1600×500 is stored exactly as you uploaded it — it is not resized or re-encoded, so a logo you had made keeps the sharpening and the colour profile it was made with.
If you set no logo at all, the header shows your site name as a wordmark. That is deliberate: a new install should look like your site, not like ours.