I'd like to play beta 1.9 pre-release 5, but most of the youtube videos I watch usually talk about the bin and replacing old .jar files. I don't see any .jar files in the bin folder, so either I'm doing something wrong or this is something you had to do before. Can still I play the beta 1.9 pre-release 5 or am I just not able too anymore?
You'll have to create a folder in the ".minecraft\versions" folder named "b1.9-pre5" and place the jar and json inside it, renaming "minecraft.jar" to "b1.9-pre5.jar". I'm not sure if you need to do anything else but the different folder structure shouldn't cause any issues with vanilla (if you want to use mods I known that older modloaders had issues with this and will likely fail to run unless you use a 3rd party launcher like MultiMC that can replicate the original environment, or possibly use the patch given here (I have no idea if it still works as it is 6+ years old). Mods that do not need a modloader should not have any issues unless they again require a specific folder structure).