Thank you for your answer

Regular PCR buffer normally added 1.5mM of MgCl2. If you want let's say final concentration of 2.5mM MgCl2 in PCR, you should add 1mM of MgCl2 into each PCR reaction. The volume of MgCl2 you need to add can be calculated using the equation C1V1 = C2V2, which will be depending on the stock concentration of your MgCl2 and the total volume of your PCR mix.

As Raditijo mentioned, it will be a lot more troublesome to reduce the MgCl2 concentration in the buffer. The easiest way to do is buy MgCl2 free buffer and add MgCl2 separately into the mix.