You can change the album artwork after uploading by submitting an edit request.
To do this, go to your album page on your eMastered dashboard and click the "Edit Release" button in the top right.
From here scroll down to the artwork section and upload a new artwork.
Streaming platforms usually process these changes within a few days, but it may take up to 1–2 weeks for the update to appear everywhere. Remember that the same artwork specifications that apply to new uploads also apply to replacements.