Can I transfer data from a Windows 10 laptop to a Windows 98 SE laptop with a serial to USB cable, and if so how?
Windows 98SE supports USB 2.0. The easiest thing will probably be to copy the files to a USB stick and then transfer them to the old computer with Win98.
The laptop has no USB
Yes, you may also need a null modem adapter or a null modem cable.
To transfer data you need a terminal program on each of the computers. Hyperterminal is included in Win98, Putty is recommended for Win10.