It's kind of strange to have someone ask about lowering MG soil PH.... most people try to raise it and complain about it being too acidic... If it was a supplement or something that was added that raised the PH too high, leach the soil.
You can use aluminum sulfate or just sulphur . Mix it in your soil like you would lime. But ya make sure you wanna lower your PH of the soil, the MG potting soil is usually acidic.