Eight steps to changing your LinkedIn background. Note, this is only possible via desktop at this time.
Step 1 - Click on your photo
Step 2 - Click the pencil icon to the right of your profile photo
You should now be on a page that says 'Edit Intro' at the top left
Step 3 - Click the pencil icon on the blue background. This is top right beneath an 'X'
Step 4 - Click on 'Change Photo'
Your office/home network page will pop up
Step 5 - Select image you want to display. I found a landscape image works better
Step 6 - Click open, Photo will upload and then you can edit how it looks
Step 7 - Click 'Apply'
Step 8 - Click 'Save' and voila! You've banished the blue background