I'm running in a machine with a 64bits CPU, but my kernel shows me (when I use uname -m command) that I'm running on 32bits (686). How can I change the kernel version to use all capacity of my machine?
You need to download and install the 64-Bit version of Ubuntu.
There is no way to do a direct upgrade from 32-Bit to 64-Bit.
You can get the X86_64 version from the Ubuntu download site here