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Above you can find previous versions of firmware.
First backup your complete nuvi including the hidden /.system folder (Wise thing to do on a monthly basis)
Then delete gupdate.gcd inside the hidden /.system folder (this file is your 7.9 firmware file)
Download the 7.80 gcd file from the link supplied ubove (nuvi24x5and25x5_GCDFile__780.gcd)
Connect your nuvi to the computer make sure that it is set to mass storage mode.
Open the Gdc file with Webupdater and it should transfer the file to your nuvi.
After it has finished you need to disconnect nuvi from PC, Switch it on and it should flash as per a normal update.
Another method is to rename nuvi24x5and25x5_GCDFile__780.gcd to gupdate.gcd and then copy the file to the hidden /.System folder.
After Nuvi is switched On it will flash.
There is a third method which is to use updater.exe and to flash in pre-boot mode. (This is used mainly for bricked Nuvi's and is not necessary at this stage)