Hi Lily,
The problem here is that the LCD and front glass are bonded together with a glue called OCA, or Optically Clear Adhesive. It generally requires specialized tools, equipment and experience to separate them successfully, and then rebond the new glass to the old LCD. In almost all cases it's not worth the time and effort it takes to do that; in order to make sure you don't get bubbles in the glue you need to cure it in a vacuum chamber, which most of us don't have at home.
Since the replacement display sells for under $25 USD on AliExpress, it's really not worth your time to mess with separating the glass, cleaning the LCD, ordering the correct OCA and then trying to get it reglued without bubbles. I strongly recommend you just bite the bullet and replace the whole thing.