Follow the instruction above your post.
Or download the file you want, rename it to keyboard.apk.
Using root explorer, make a copy of keyboard.apk from /system/app. Put it somewhere safe (sd card or computer).
Get the downloaded keyboard.apk and put it in the same directory as above (first, press the r/w button on top of the screen when on /system/app).
change properties to 644 (read/write for user, all others only read) using properties of file.
Reboot (not sure about this part, but it never hurts).