I am trying to download a firmware update for my nikon d850 but the download is in a zipped bin file and my imac cannot open it any help would be mutch appriecated

This is from Nikon Support,

1. Download F-D850-V120M.dmg.
2. Double-click the F-D850-V120M.dmg icon to mount a disk image containing a folder named “D850Update”, which in turn contains the following file:
3. • D850_0120.bin (the camera firmware)
4. Using a card slot or card reader, copy “D850_0120.bin” to a memory card that has been formatted in the camera.
5. Note: Be sure to copy the firmware to the root (top-most) directory of the memory card. The camera will not recognize the new firmware if it is placed in a folder under the root directory.
6. Insert the memory card into the slot currently selected as the primary slot on the camera and turn the camera on.
7. Select Firmware version in the SETUP MENU and follow the on-screen instructions to complete the firmware update.
8. Once the update is complete, turn the camera off and remove the memory card.
9. Confirm that the firmware has been updated to the new version.

Note: For more detailed instructions or information on the equipment needed to complete the update, download the following pdf file:

DSLR_Firmup_Mac_En.pdf (PDF) (0.17 MB)

Note: Updates can be performed for you by a Nikon-authorized service representative.

Here is the link to the Nikon Download Center with firmware for other model Nikons and complete details on what the firmware fixes and added features they may contain.

https://downloadcenter.nikonimglib.com/en/products/359/D850.html

I hope this helps.

You need to download the Mac version which is a .dmg file!