Purchase a W98SE cd disk. They cannot be downloaded, and using the Windows
Update feature will not convert a W98 OS into a W98SE OS. Microsoft no
longer markets W98SE, so they are scarce. Try eBay, but be very careful
which version you select.
I recommend a RETAIL (not OEM) version, and there are 3 different ones: the
UPDATE cd (cheapest, but requires that W98 first edition already be running
on your machine), the UPGRADE cd (requires proof of purchase of a previous
Windows OS. Proof is established by simply inserting the floppy or cd disk
when requested to do so during the W98SE installation process), and the FULL
INSTALL cd (no existing OS required).