1st of all, you need to be on maemo 5 and have a text editor such as leafpad (my preferred, more user friendly than vi) installed and of course you need to be in root to do this ! ;)

run a xTerm session

root (or sudo gainroot, depends if you are in RD mode or not, or if you have installed another package to get the root access)
mount -t ext3 -o noatime,rw /dev/mmcblk1p2 /and
leafpad /and/system/build.prop

then search for the line "f.lcd_density=160"
and replace it by "f.lcd_density=240"
(without the quotes - by the way, if it's too big for you, you can enter any number that you want but i would not recommand you to go beyond 240)

after that save it by clicking the upper contextual menu

exit leafpad

exit xTerm

reboot your device and voilaaaa !