You can't upgrade from WinME to Win98 SE because switching from WinME to Win98 SE is a downgrade. You'd have to do several different very technical procedures, either formatting HDD and installing Windows or, partitioning your HDD and installing Win98 on that newly added partition creating a dual-boot system with both operating systems (OS) on board. I'd stay with WinME if you can.
Tell the forum the particulars and maybe we can help you to stay with WinME or, if you want to take the big step, backup all your sensitive data on your computer's hard drive (HDD), format that target HDD and install Win98 SE from scratch.
I'll include a Starting From Scratch tutorial just so you can print it out a see what's involved. It would be a very big step to start all over.
Please state the brand of computer you're using. Most computers come with some sort of Restore /Recover CD(s) that let you put your computer's software back to the way it was new out of the box. Likely, this forum'll resolve your issue in one way or another.