Answer: by scraping it with a chisel-type tool or by sanding the ground.
When you decide to replace an old tile with a new one, some of the old glue often stays on the floor. Two options are available to you. If the glue is solid and compact, you can very well leave it on site and smooth the floor over it before laying the new tiles. If it tends to crumble or crumble, you will have no choice but to remove the glue completely by scraping it off with a chisel or any tool doing the job available to you. Or by sanding the floor. In this case, there is no need to make patching of course. Then, the entire surface must be thoroughly cleaned to remove any debris and dust deposits to ensure good adhesion of the new tiles.