If you want to wipe the drive clean and then install 98, complete instructions are on
this page.
While the instructions assume you start with Win98, the process is the same when starting with a WinME installation...boot to boot floppy, fdisk, format, install Windows.
That is the best way to change operating systems but you will lose all your data...documents, email, address book, pictures, music, etc. So if you don't have everything you want to save backed up to CD, DVD or another drive, do that first.