I have dual OS (RHEL 5.4 & Ubuntu 10.04) installed on my system. Now, I want to upgrade to 11.10 or 12.04. How can I upgrade using Update manager? Will that affect my dual OS boot?
If you are managing your dual boot through grub2 i.e. the boot software installed as part of Ubuntu, then upgrading through Update Manager will not affect other operating systems installed.
If you have installed boot software through RHEL then this is off-topic for this forum and you should flag this to be moved to Unix & Linux stack forum.
With regards to your upgrade to 11.10 from 10.04, you cannot upgrade directly through Update Manager. You will need to upgrade in steps - 10.04 to 10.10 to 11.04 and finally to 11.10.
It is not recommended to move to 12.04 - it is still in early alpha state.
If you want to help out the testing effort - upgrade as explained above to 11.10. Then in a terminal type