When u use the mirror modifier you need to select a vertex on the edge of the object and make it the new center of the object
what’s the command to make it the new center
Where is it located in the menu
Then you can apply the mirror modifier
Tanks & Salutations
You can use shift-s to snap the 3d cursor to location in which you want to place the object center. To set object center to that location, use Buttons Window\Editing (F9)\Mesh\Centre Cursor button while in the Object Mode.
So there is nothing in Edit mode to re-center !