what is a cover?
If you click on my avatar, the desert scene is a cover. It also shows if you go to my Profile page
Now to set a Cover, go to your Profile page
Under the line that shows your last activity, there are two red buttons
1 is Find and the other is Cover
If you click cover you get
modify cover image shows
which are self explanatory
Style Cover - Lets you choose from a colour wheel
Choose cover Preset - This is a feature that will be enabled at a later time
Delete Cover - Self explanatory
It lets you make your profile feel more at home and spices up your avatar in posts.
If you click on my avatar, the desert scene is a cover. It also shows if you go to my Profile page

Now to set a Cover, go to your Profile page
Under the line that shows your last activity, there are two red buttons
1 is Find and the other is Cover
If you click cover you get

modify cover image shows

which are self explanatory
Style Cover - Lets you choose from a colour wheel
Choose cover Preset - This is a feature that will be enabled at a later time
Delete Cover - Self explanatory
It lets you make your profile feel more at home and spices up your avatar in posts.