i need to know how to make a update.zip on sd card so i can boot from it just in case

[alan2121m]

man i want to back it up because last one i bought was defective it would boot to the virgin mobile logo screen and wouldnt go pass that, and i did a hard reset and it gives you the option to boot from sd card that has to be named update.zip so i've been trying to figure out how to get it or make that update.zip file, just in case it happens again its frustratiing my phone is a ZTE Virgin Mobile PCD Chaser