Don't use ROM Manager at all... It's just gonna cause you nothing but problems.
OK I'll try and walk you through it.
Make sure you have SuperSU installed.
Download the v10F nandroid backup to your sdcard.
Download the L9 Recovery Installer from here -
[LOCKED BL/Jellybean support] LG Optimus L9 Recovery Installer | v1.5.0.0 27.04.13 - xda-developers
Download BusyBox by stericson from the PlayStore.
First
Install the BusyBox APK, then open it up and click on the version of BusyBox to be installed drop down window and choose v1.21.0 to be installed to /system/xbin/ and install BusyBox.
Now go to the L9 recovery installer APK and install it.
Open the recovery installer and click on install recovery and the choose "ICS/jb stable" grant it SuperSU permissions and once it's finished installing the recovery, select reboot now.
When the phone reboots the front keys will start flashing rapidly. While the lights are flashing, press and hold down the volume (-) down button until it reboots to CWM recovery.
I'd recommend at this point to make a nandroid backup of your current setup just incase you need it.
Just choose backup and restore and then choose backup to sdcard.
Once the backup is done.. Wipe cache, go advanced and wipe davik cache and then wipe data/factory data reset.
Then choose install from sdcard.
Navigate to sdcard/download and choose the v10F nandroid backup and click on install.
When it's done installing select reboot and you'll have a fresh new phone on v10F.
Also the v10F nandroid is uploaded to Mega. It can be a pain to try and download stuff from there. What I've found is when you go to the Mega site, it will ask if you have the mobile app, just hit cancel and then press the menu button (right hard key) and select "Request desktop Site" then the Mega site will load properly and give you the download link.