Step 1:
Locate the retweet you want to undo.
Step 2:
Click or tap the "Retweet" icon
.
Step 3:
Select "Undo Repost".
The "Repost" icon will change back to gray.
How to Delete a Quoted Tweet:
Step 1:
Locate the quoted tweet you want to remove.
Step 2:
Click or tap the "More" icon 
in the top right corner.
Step 3:
Select "Delete".
Step 4:
Confirm the deletion.
Please note: The action of deleting a quoted tweet cannot be undone. It will be removed from your profile, the timeline of any accounts following you, and from search results.