If you know how to add - it is enough to perform the subtraction:
M = P - Q = P + (-Q) - here M point is P point minus Q point, which is the same as P point plus (-Q) point. So you make the addition of points P and (-Q).

If Q point has coordinates (Qx, Qy), so (-Q) point will have coordinates (Qx, modulo - Qy)

Now you can easily can make a subtraction:
M = P - Q = (Px, Py) - (Qx, Qy) = (Px, Py) + (Qx, modulo - Qy)