[General Questions and Answers] Droidriven:
Download and install Odin 3.09 for windows on your PC, I assume you already have the USB drivers for Samsung installed.

You will need to flash a custom recovery to the device in order to flash the custom ROM you have selected, it will not flash through the stock recovery, do a Google search for "CWM touch 6.0.4.5 for GT-i9300" or "CWM touch 6.0.4.7 for GT-i9300", make sure that the file you download is in .tar or .tar.md5 format, you do not want the .zip or .img formatted files.

Once you have Odin installed and the .tar format recovery file downloaded to PC, make sure you have the custom ROM you already have and its compatible Gapps stored on an ext sdcard and the card is in the phone(if you dont already have a Gapps file with the ROM you downloaded then do a Google search for them, you will need to match the version of Gapps to the Android version of your ROM, 4.4 ROM/4.4 Gapps, 4.4.4 ROM/4.4.4 Gapps, 4.4.2 ROM/4.4.2 Gapps, 5.0 ROM/ 5.0 Gapps)......make sure your battery is fully charged boot the device to download mode by starting with phone powered of and pressing and holding the volume down, home and power buttons at the same time, when you see the warning screen press volume up to enter download mode, with device in download mode and Odin open on PC connect device to PC, if its recognized in Odin you will see a blue or yellow comm box, if you don't see this then try a different USB port and make sure USB drivers are installed properly and that you are using the stock Samsung USB cable. If you are recognized then click the "AP" button in Odin then browse to where you downloaded the recovery .tar file on your PC and select it, when it takes you back to the Odin screen look for the Autoreboot option and make sure it is UN-checked, then click start in Odin, the recovery will then flash, wait for the green PASS then when its done boot straight to recovery by pressing and holding the volume up, home and power buttons, once in recovery you can make your selections by touch if you got CWM touch, select factory reset/wipe data, then OK or yes, when its done select wipe cache partition, then OK or yes, when its done then select the advanced option, then select wipe dalvik cache, then select go back, on main menu in recovery select install zip, then select install zip from sdcard1, then browse to your ROM.zip and select it then select OK or yes, when its done select install zip from sdcard1 again then browse to your Gapps.zip and select it then OK or yes, when its done select go back then on main menu select reboot now.

It will boot and be slow the first time, when it finally boots go through setup wizard to setup phone and enjoy.

If you have issues with service or data then the update you originally did may have corrupted data connectivity in some way, sometimes the factory updates are actually for another device model and breaks things when it updates. If so then you will have to download the GT-i9300 stock firmware, I'm sure its available for download in many places and a simple Google search will find it, when you have it flash it through Odin the same way you did the recovery .tar as described above but this time you'll have Autoreboot checked instead of unchecked, follow the same steps other than that. Once its flashed, booted and setup you can leave it that way or you can start over with rooting it, and installing the custom ROM you have (don't do the update that you did the first time). To install your custom ROM you will have to flash the custom recovery .tar through Odin again then flash the ROM and Gapps as described above. Come back here or PM me if you need more help, I've tried to give you what you need and a contingency if your original update broke anything that the custom ROM zip won't fix.

Dripped from the Tapa of my (Deathly)Liquidsmooth S3

---------- Post added 14th February 2015 at 12:14 AM ---------- Previous post was 13th February 2015 at 11:58 PM ----------

In case you are missing NOOKIE's point, he's saying that if you take it back in its rooted form then no you will not be under warranty but if you unroot and restore back to stock firmware then they have no way of knowing that you rooted your device, one more time, JUST UNROOT AND TAKE IT BACK!!!!! got it THIS time?

Dripped from the Tapa of my (Deathly)Liquidsmooth S3