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v Profile ➡️ Profile Banner ➡️ Edit
^^You can then choose a different banner if you'd like. Keep in mind: The dimensions of a profile banner matter, and if it's custom, some things could be hidden/cut off. (A perfect example of this is my profile banner, haha.)
The suggested dimensions are: 1070 x 175h (width/height), but you can use one that's any size, iirc.