How do I change the logo on my site?

The logo on my sites is currently a blue square with an abbreviation of the site name.

I have tried to change it a number of times, but it just doesn't work. I go to Site Information - then Change logo and add a logo from my computer (apparently I can choose a picture from 'Site assets'. I press Save  and the logo changes briefly to my uploaded logo. However, when I refresh the blue-square logo is back. How do I fix this?