The design of your proposals contains a similar theme with your navigation on the left side with your logo at the top and your content on the right.
Changing your proposal logo
Hover over your profile picture in the top right corner of your Dashboard and click Settings.
You’ll be able to choose the brand you wish to edit under ‘Branding’ section and find the Sidebar settings in there.
Click 'Browse for logo' and replace the current one with your own. It's best to use a transparent PNG with a minimum width of 400px.