Install the new version of Ubuntu (presumably 10.04, or 10.10, I'd recommend the former), on top of the existing Ubuntu 9.10 partitions. To do this, insert the boot disc for the new version, and install as normal until you reach disk partitioning. Select "manual", and remove, or "format" the partitions currently used by Ubuntu. Be careful not to remove or format your Windows partitions.
It's worth noting that, the practise of keeping a separate
/home partition is useful in these situations. Furthermore, should you wish to temporarily keep 9.10 around, or you are happy to install the new Ubuntu side by side with the other operating systems, you will have no problem doing this. The most recent version of Ubuntu installed will be listed at the top of the boot menu.