If you made mistake to this file, you phone will not boot in gui, so you should always backup the file before you make any change to it. Just use any file manager to copy the file to your sd card or your pc.
If you have a Linux machine you can mount the /system partition while plugging in with the purple screen (vol up + vol down + power).
Easiest way: get Ubuntu, put your phone in purple screen, plug in with USB, you will see the system partition at your Ubuntu desktop.
The hard way: if the /system partition is no accessible under purple screen mode (such as in the latest 528 rom), you can use adb push the backup file back, search the web for details about how to use adb.
I'm digging the build.prop, some apps can do some of these tweaks for you, but I prefer typing myself. You need to reboot the phone to take effect.
add this new line to make UI more responsive, the bigger the value the more responsive, I've tried 90, too fast, fall back to 75, still quite fast
change value from "false" to "true" to enable menu key unlocking screen
change to 225 to increase the resolution of display, as a result, the size of icon on home page and fonts of list (of file manager or contact) become smaller, the smaller the value the smaller size of icon and fonts
increase the value to 32m~48m to increase stability and performance, tested but doesn't see much effect so far