Hover over the Cogwheel and navigate to Account > Settings.

Select General.

Here you can select Choose File under Logo.

To add your logo into a document, you will start by navigating to the Cogwheel then select Account > Documents.

Select the pen and paper edit option on the right hand side of the document you wish to add your logo to.

Add the variable {LOGO} to the document.

Lastly, save your changes for them to be implemented.