need
-root explorer
-whatever font you want. the regular version and a bold one.
in root explorer press the mount button at the top right so that it says mount r/w instead of r/o. navigate to whatever font you want (it would help if they were in the same folder). rename the regular version "Arial.ttf" and name the bold version "Arial-Bold.ttf" both without quotes, of course. then hit the menu soft-key, choose multi select, and then tap both of those files to choose them. then hit copy and navigate to /system/font folder (make sure it is mounted r/w). now you will hit paste. once you've done that you'll see the two arial fonts will have different permissions than the rest of the .ttf files in the folder. long press each one (one at a time) and change permissions to user: read, write; group: read; others: read. it will end up looking like this rw-r--r--
and there you go, you've successful changed your system font. reboot the phone and you're done.