This one may or may not be easy. First is the cord to your computer RS-232 with a 2nd interface to USB or is it USB direct?
Which version of Windows you have makes a difference.
There are two major makers of RS-232 to USB interface chips Prolific and FTDI if I recall correctly and there are some counterfitters doing FTDI chips as well.
Now.. If you have a GENUINE FTDI chip, Skip to the "Fixing" part of this post.
IF you have an Immititation (Counterfit) FTDI chip, and windows 10 (not sure about windows 8) it is a brick,and will never work again, Sorry but that's what you get for saving a few bucks.. NOTE: if the chip is in a bulge in the cord between the Garmin and the Computer (As it is with a piece of medical gear I have) a new cord may fix it, Make sure it is a GENUINE FTDI chip.
If you have a prolific chip. Well then the chip is still good, Just no driver for Windows 10 (And I think 8) The medical hardware I had used one of these, They sent me a new cord, no questions ask other than "Wher do we send it?".
Fixing:
Windows will install the RS-232 port willy-nilly May be Com 1,2,3,4 27, 32 whatever.
Open DEVICE MANAGER (On windows 10 right click on the windows icon (lower left corner) and choose device manager.. On older versions of windows pick control panel hardware device manager (if I recall the path correctly)
Scroll to Ports (Com and LPT)
Find your RS-232 adapter
Now from here on the path varies but click on any "Details" or Setting option and find the PORT settingsk, Make it one the software can see (Generally 1,2,3 or 4 for the software) and see if that works
Check the software first, Some programs do not have a problem with Com-15 Many do.
Oh, one other item: How to tell what kind of chip is in the adapter...
Follow the "Fixing" process, only change nothing, The Device manager should tell you the maker of the device.