The only way to remove an acid stain is by grinding the stain off the concrete. A lot of times, especially with the darker acid stains, even after grinding the concrete you will see a hint of color still in the concrete. Once you have ground the old acid stain off the concrete you can re-stain the concrete with an acid stain or a dye.
Here we are grinding an acid stain off a concrete patio. After removing the acid stain we will use an acetone dye to color the concrete.