how do i re align the headlights on my car?

To be honest, most decent local garages will do this for free (at least mine did).... depends on the model of car, but there are usually adjustment brackets at the back of the light fitting - if you point the car in front of a wall (so you can see where the lights are pointing) and adjust the new one to match the old one.

Remember, the light that will be nearest oncoming traffic should point marginally lower than the "off side" light

There are screws on the top and side of each headlamp(there should be only two, one at the top, one at the side). They are slightly recessed and the heads of the screws are either hex heads or allen wrench type(they are usually under the trim depending on the model). Use a wall on level ground to align. Turn on the lights and aim at the wall and adjust as needed.

Park in front of a wall or similar and re align them like that.

You can park in front of a wall and mark were the centre of the lights are on the wall the low beam will be just below the line and high beam will be on the mark or very slightly over. you can adjust them by screws on the top or back of the lights.

Take it to Kwik Fit (or similar) and ask them for a Pre-MOT check. They should align them for free.

Park with a wall in front and realign them like that

there's usually just a single screw that you turn to alter how high the beam points, should be kind of obvious looking and maybe have an arrow or marking next to it, just twiddle any bolt that doesn't hold the thing in place.

Pop down to your local garage and ask them. It takes seconds to do and they may do it free, hoping you return to them for car servcing etc..