you have to have a custom recovery installed on the phone.
download of these:
https://www.mediafire.com/folder/8uuvfvauztdna/MEGA_6.3#wyopegjd4377g
fallow these instructions:
How to install Recovery using odin:
Download mode: you do this by turn off phone, hold down the power - volume down and home keys. it will vibrate than a WARNING!! screen comes up. Press your Volume up key to get into "Download Mode" than plug the phone into your computer.
Odin:
Dev-Host - odin3.09.zip - The Ultimate Free File Hosting / File Sharing Service
Installing Recovery through odin: open odin, click the "BL" button find your recovery.tar select it. Click start. let odin do its thing. You should now have recovery!!!
To get into recovery after installing it, you hold down the power/volume up and home key. you'll see samsung galaxy splash screen with blue recovery in the upper left corner let go of the buttons. it will to into the recover. you'll see a number of selections. you want to select backup. Make sure you back up to an external sd card. to flash a new rom after. first select wipe. after it wipes than go back to the menu and select install. find where you put the rom's zip file and select it.
if you are in the homescreen you can use an app called quick reboot to get into recovery.