Open the song and scroll down to the Metadata section. Click "edit" beside Song title to open the Edit this song popup. You can fix a typo in the title or an artist's name there, and also do the following.
Collaborators
Click "Additional artist (for collaborations)" and pick one of your artists or add a new one. To take someone off the song, click "remove" next to their name. "Swap with main artist" makes them the main artist instead.
Remix or cover
Tick "Is this a remix or mashup?" or "Is this a cover song?", then pick the original artist. They become the main artist and you move to the remixer or cover artist slot. Picked the wrong one? Tick the other box to switch. To undo it, untick the box and choose which artist stays on the song.
Official release
Once a song is marked as a remix or cover, "Is this an official release?" appears in the same Metadata section with Yes and No. Click either to change it.
Submissions you've already sent don't change. Any edits apply to what you send next.