I have a 2gb chromebook with 64-bit ubuntu 12.04 and it runs extremly slow and often crashes so how to I downgrade it to 32-bit or is their another solution?
It is not possible to convert an existing installation between 32-bit and 64-bit while keeping installed software. You need to do a reinstall.
The reason behind this is that it's actually classed as a different CPU architecture. Despite the fact that the CPU can run either, and Linux distributions are starting to support multi-arch (and you can even run a 64-bit kernel with a 32-bit installation), you still need to choose one or the other for your base architecture and it's not something you can incrementally swap over on a working installation.
Note too that the assumption behind your question is false. If your computer is crashing on 64-bit it is almost certainly not because it is 64-bit. While there is always a possibility that a full reinstall (whether moving to different version/architecture or not) may solve a problem - particularly if you didn't have the problem when you first installed - it represents a last resort in troubleshooting.
You'd really need to completely reinstall, though if your /home resides on its own partition you could keep it.
I'd be doubtful it'd give much of a boost, especially since while 64 bit is a bit hungrier on memory, it's not that much hungrier and in some ways it's faster.
I'd try tweaking as per the various performance threads here, but on a low spec machine I'd lean towards the lighter desktop environments such as those that come with lubuntu or xubuntu.