I am new at modding and i am in forge My question was how do i modify an already existing block property, like for blocks that you create its easy:

public static final RegistryObject<Block> BURNEDWOOD = registerBlock("burnedwood", () -> new Block(BlockBehaviour.Properties.of().blablablabla));

but what do I put in for just vanilla blocks(I want to change the oak log to take more time to cut down, but idk how to access the properties of said block)

You can't change vanilla. The best you can do is create a custom block, and inherit your block from the vanilla one