This is what you will have to do if your bootloader is locked and you are not rooted (and assuming you are currently running yakju ICL53F):

1) Download the yakju-IMM76D-from-ICL53F update from here, and place it in /sdcard on your device.
2) Follow steps 1 through 6 (inclusive) from here, but in step 3, you only need to push the mempodroid file, not all three files.
3) Once you see the #, there are two commands that you have to type:

cat /sdcard/7f97fbc19417.signed-yakju-IMM76D-from-ICL53F.7f97fbc1.zip > /cache/7f97fbc19417.signed-yakju-IMM76D-from-ICL53F.7f97fbc1.zip

rm /data/local/tmp/mempodroid

4) Close the command prompt and reboot your device into your stock recovery.
5) When you see the Android lying down, push volume up and power together repeatedly until you see the recovery menu.
6) Navigate the menu to apply the update.

Notes:
1) You are not rooting your ROM doing this. mempodroid just grants the shell user temporary root access to your system, that is all. You will have no root as soon as you close the command prompt.
2) Doing it this way (i.e., with a locked bootloader and no root), you CANNOT create a backup of your current ICL53F ROM, in case you do not like the IMM76D ROM.
3) I highly recommend to everyone to unlock the bootloader and be done with it. You can always re-lock it (unless you break your USB port) in case you need to return the phone to the store. An unlocked bootloader has no impact on the running of the ROM, nor on receiving updates, nor Google Wallet, nor anything else. But it does give you the ability to use fastboot commands which are EXTREMELY useful and can save your butt in many cases.