Unfortunately, you cannot directly apply unsanded grout on top of the grout already there unless there is enough depth. If there isn’t, you’ll first need to remove the grout already applied and then apply the new one.
To do this, use a thin blade like a plexiglass cutter. A grout removal blade can be thick and scratch the edges of your tiles. Run the thin blade in the grooves until it’s all out, then wipe it down and apply sanded grout.