Use the X resources documented in the XTerm man page, particularly the "ACTIONS" section about 80% of the way down.
Create a file ~/.Xresources and put in it:
XTerm.VT100.translations: #override \ Shift <Key>Up: scroll-back(1) \n\ Shift <Key>Down: scroll-forw(1) "Translations" is the base Xt library's name for key and mouse bindings. This sets bindings for the VT100 (terminal emulation) component of XTerm, overriding any existing bindings and setting Shift-Up to scroll up (or "back") one line, and Shift-Down to scroll down ("forw"ard) one line. We're making one long line so we're using backslashes at the end to mark continuation.
As with any use of .Xresources, you will need either to have the XENVIRONMENT variable set pointing to the right place, or use xrdb -merge ~/.Xresources to load the file into the resource manager explicitly.