You would not be able to add or change a background photo from your LinkedIn mobile app but you will be able to do the same from a desktop.
Delete a background photo in LinkedIn
Open LinkedIn homepage by entering your login details. Then click on the Me icon found at the top of the homepage.
Now, click on View profile.
Then find Update background photo at the top of the profile and click on it.
If you want to reposition the photo then click on Reposition which lets you drag and reposition the image. If you are looking to remove the photo then click on Delete.
Click on Save to save your settings.