reinstall of original bios after installing the modded one on dell xps l502x

If you updated the bios and after the reset, or restart the laptop no longer boots and you are met with a black screen you have bricked the laptop.

There for you cannot get to the stage of performing a flash back to the old bios version that the laptop was working with if the case.

The file states the model revision of the laptop.
And it has to match that exact laptop.

There are slightly different models in the range, based on the revision.
get it wrong and you render the laptop completely useless with now way to fix it.
If it worked ok as it was you should never install a modded bios firmware file.

Does the laptop boot after the bios firmware update.
If the answer is no.
Then you will have to pay someone to open the laptop de solder the old bios chip and fit one with the right firmware version to get it working again.
That will cost some money.