To change the transmission oil in an automatic transmission, first locate the transmission pan cover, place a large drain pan under it, and loosen the bolts on one side of the pan until they come off. Tap the transmission pan to help break the seal. Slowly remove the remaining bolts, and drain the old transmission oil into the drain pan.
If your transmission features a drain plug, remove it to drain the old oil. Once the transmission oil drains, carefully remove the old gasket. Clean the oil pan using a clean lint-free cloth, and replace the transmission filter. If the filter is metallic, clean it with a solvent, and reinstall it.
Attach the new gasket to the clean transmission pan, ensuring it's in the correct position and that the bolt holes are properly aligned. Insert the bolts, and tighten them as per the torque specification. If you removed the drain plug, reinstall it. Fill the transmission with new transmission oil. Consult your owner's manual for the correct type and quantity of transmission oil.
Start the engine, and move the transmission selector through all the gears to circulate the new oil, then check the new oil level. Adjust the level if needed.