This can only be done on the web - so grab a computer! You can't change your image on the app yet (but you can change your title!!)
Open your collection that you want to edit.
You'll see a LIKE, SHARE, and EDIT button on the bottom of your cover.
The Edit button will give you some option and you can either click the big red "Edit Cover" button, or the small drop down menu.
Other important fact: you can click Publish Option and put KPOP or whatever community your Collection is for, so it will show up listed as a Kpop collection in the community :D
On the top of the Edit Cover page, there are some buttons for you to edit it.
Click the one that looks like a picture, and you can upload whatever you want!
If you made your own cover and it already has your title on the image, you can make the other text invisible.
Click the last square on the edit box, and that is your text color - the very bottom choice is 'Transparent'