Imorted it as an AI file, want to save it as AI too

Using Linked Assets to import an Illustrator file doesn't allow for saving the file from Adobe Express as an Illustrator format. You would make your edits in the original Illustrator file, save and use Share from Illustrator. 

Using Linked Assets is more for the ability to bring your own designs into Adobe Express to format for social media etc but any major edits and work would still be done in the original program e.g. illustrator or Photoshop.