Raspbmc ships with a stable release of XBMC. But, if you are adventurous, it is pretty easy to try the latest nightly release of XBMC on your Raspbmc.
Go to Program → Raspbmc settings
Choose Nightly Build Configuration → Install XBMC nightly
The nightly releases of XBMC will be listed here, including the one which shipped with Raspbmc, which is listed as xbmc release
Choose the XBMC nightly you want and it will download and install. After the install you will be asked to reboot to switch to the nightly.
Your original or older XBMC releases remain and you can switch to them in Raspbmc settings using the Switch between versions of XBMC option.
Tried with: Raspbmc RC5 and XBMC rbp-20121231