how to change the ballaster in a light?

Determine the bulb type and wattage, this info is printed on the fluressant tubes. The type ballast (T5, T8, T12) single or double socket electrode wire tube and then get the ballast for the wattage tubes used in the fixture. High output -20F cold weather T12 ballasts (single nub) are different from the T12 indoor (2 nub) connector socket bulbs. They will have a conversion chart for the new electronic ballast replacements at the home store. Don't be surprised if the new electronic ballast is smaller then the original transformer ballast was. The wiring diagram is on the ballast.

I suggest you look up the ballast by the bulb type and then see if it's available. Home Depot used to carry them all but now they have a very limited selection. I had to source my outdoor F48T12 40 watt cold weather ballasts at the local electrical supply warehouse, indoor ballasts should be far easier to source but it saves gas to make sure they are in stock at the home supply store before you head there looking due to the fact that those home store idiots know less then nothing like Sargent Shultz. See the video below on how to convert 2 prong sockets over on the new single wire electronic start HV ballast that doesn't use the 2 prong heater filament to source the tubes free electron flow up in the tube.

https://www.youtube.com/watch?v=E456CwPq1sI

1 I.D the type of ballast you need or you can lug the old one with you i suppose so remove the light diffuser remove the bulbs then remove the humped shaped cover now the ballast is exposed Disconnect the Black Or colored wire first . then disconnect the white wire then where each light goes into the lamp holder (Its called a tombstone) The white clips on the very end where the pins or pin makes contact to power the bulb Cut the wires that go to each tomb stone usually red and blue and or yellow leaving about 6 to 8 inches cut all of them at that same length .When your done with that unscrew the ballast and take it with you or write down the info on the sticker and match it exactly if you can Then the wires you cut on the tomb stones strip them and then go color to color wire nut the connection you may find that there is a quite a bit of slack in the wires you connected its ok in a neat fashion bundle up the slack making sure its not directly up against the new ballast put the bundle a little farther down the extra space in the ballast cover Then you will hook up the white wire and then finally the black or colored wire BUT FIRST if you can shut off power to the light at your panel or make sure the switch is at least in the off position BEFORE starting the removal of the old ballast and that it stays in the off position

I did what Roger said but I tested the power after turning off the electric just to make sure. I also put pieces of tape on each wire so that they would help me remember where they went. Such as , A+ A, B+B. With my memory I know I need extra help. It is also best to take the old one to the Hardware Store with you so you can get the exact match. It is not hard to do and the only trouble I had was keeping it in place while I screwed it back on. I have tiny hands and the ballaster was quite heavy for me to hold over my head. I got it fixed though!

turn off power

remove the light diffuser

remove the bulbs

take off the over over the ballast

cut the wires to the ballast

unscrew the ballast

screw on the new ballast

connect the wires from the lamp sockets to the wires from the new ballast (match colours) use wire nuts

replace ballast cover

replace tubes replace diffuser

turn power back on.