Update: I could not download the file with the link in the following paragraph, for now I have to stick with second-to-last release of BeShell, which works fairly well for me. In order to get it, go to be300.org site and then go to BeShell section and the link will be in one of the upmost threads. But first try to get the latest BAS beshell which is linked below, it may work for you!
In order to get the latest BeShell, you need to sign up for an account at be300.org. The thread where the latest version is posted is: BAS BeShell thread
The reasons for getting BeShell are that it supports Russian text viewing (and writing), it supports many more programs and features that standard BE OS, and it generally looks and works nicer. It looks more like the OS for the more expensive Pocket PCs.
There are a few ways to install a new OS on BE-300:
Perhaps the easiest and the one I used is to download .bin backup file of the OS in question and to use restore from backup feature by holding OK and pressing reset button with your stylus. After that you choose to restore from the PC, and pick the .bin file you downloaded.
The other method may be even easier and the only difference is that you get .dat file, and put it in your CF card, in My Documents/Backup. I haven't tried this method.
With both of these methods, you lose everything on your Nand disk, but data in your CF card remains intact.