Before I answer your question: Do you have any special reason why you want to increase it above that size? If you are not sure, do a search here on linuxquestions.net and you will see that 2 GB (with another 2 GB of RAM) should be more than enough in most cases.
Before you delete your old swap turn it off so that the system doesn't try to use it anymore. Read "man swapoff", you should do something like "swapoff /dev/hdc3" (device file depends on your setup!).
You will have to delete the swap and create a new one, cfdisk is a command line partitioning program that can help you do this. Make sure you set the type of the newly created partition to "82" and check that cfdisk shows it as Linux swap space.
Create the swap: read "man mkswap" -- for example to create a swap partition on /dev/hdc6 to "mkswap -c /dev/hdc6" as root.
Turn on swap: read "man swapon", something like "swapon /dev/hdc6" should be ok.
Change your /etc/fstab so that your system will use your new swap space by default: Edit this file as root and substitute your old swap's entry device file (first colum of /etc/fstab) to the new one.