Sounds like you have downloaded it since it seems like you're saying you see _____v1.005_eng.zip
You should be able to click on that and unzip the file, which should open a folder with blackvue files in it. (You might want to click view in finder first, as suggested above - just makes it easier to know where the file was saved)
Plug your SD card in (you should probably always format the card first before you upgrade the firmware)
Not sure what size of card you have but you can try to open the PC app, BlackVue Viewer.
(You should have saved any recordings you have on the SD card to your computer by this point)
Click the SD card format button on BlackVue Viewer (near the top right, looks like a micro sd card with 2 circling arrows on it)
(you could right click on your SD card to format it as well, sd cards 64GB and up may need some help from a 3rd party application)
Now that it's formatted you can copy the "BlackVue" folder that you recently unzipped from the firmware you downloaded and copy it onto your SD card, replacing any current "BlackVue" folder that may be on the sd card (though since you formatted the card, there should be nada on it)
Plug the SD card with the new (old, in this case) firmware back into your dashcam and power it up.
The camera will apply the update and you should hear a prompt when it's done. It'll reset once it's done.