Max says:

Hi Richie, most locomotive bodies are either held on with screws on the bottom of the chassis, or they are held on by plastic tabs on the side of the chassis. (Note: Before starting to disassemble your locomotive, the locomotive’s box will tell you if it is DCC ready or not, so check it first if you still have it.) However, as most steam locomotive models are oddly shaped, their bodies will most likely be held on with screws. First, try to locate the product on Bachmann’s website. If you can’t find it on their website, turn the locomotive so you can see the bottom of it. There should be multiple screws holding the chassis and body together. Take a screwdriver and unscrew all of the screws, keeping them somewhere safe, and remembering which screws go where. After you have unscrewed them, try removing the chassis from the body. Be very careful when doing this, as the chassis might come apart because the screws holding it together have been removed. You may have to remove any additional screws the chassis was covering up as you take it off. Once you get the chassis off, you
should find the chip on it or in the body. Remember to put the screws back in the reverse order you took them out (last to first). I hope this helped, and happy modeling!