Do you mean the Dock, or do you have an alias in the Sidebar? --I don't know how that is possible.
Open a Finder window and open the Applications folder (cmd-shift-A).
Look for Photos and double-click to open it.
For the Dock…
If that opens, drag the old alias out of the Dock and hold until you see the "Remove" label appear--drop it.
Drag the open Photos icon from where it is in the Dock to where you would like to have it in the Dock. Make sure the other icons move out of the way, then drop the icon in place.
If you had an alias somewhere else, please explain exactly where and I'll try to give instructions on how to update that alias.