Rearrange the function so the y or dependent variable is by itself. For example, in the equation y + 2 = x^2, rearrange so that you have y = x^2 -- 2.
Substitute (x + a) in your equation for x, or the independent variable, where a is the number of units you want to shift to the left. For example, to move y = x^2 -- 2 three units to the left, substitute (x + 3) for x so that you have y = (x + 3)^2 -- 2.
Graph the original equation and then graph the translation in Step 2 to see the shift.