Ho do I import a dive log form another dive computer in either garmin dive or garmin connect

i don't think you can. you can do it in another dive logging app only.

It can be done but it not easy.  I know someone that did it.  Basically you'd have to convert each dive into a FIT file with the correct format and then import each one into Connect.  The person that did it, didn't even have a Descent.  I sent her a couple of my FIT files and she used some of the FIT editing tools to reverse engineer the format.  She was only doing to earn badges.

That being said, I'm not sure the value of going to all that trouble.  Both Connect and Garmin Dive are pretty poor as dive logs.  They only display very basic information about the dive.  There are so much more data captured by the device that is not available.

An app like Subsurface supports most if not all dive computers, works on Windows, Mac, Linux, iOS and Android.  It even supports multiple dive computers for the one dive.  For the Descent, it displays most of the data in the FIT file, including gas switches, NDL, ascent rates, aletrs.

I use Subsurface but have not been very happy with it.  Found it to be buggy and no good share to social media feature. Also use Shearwater cloud for my main computer and and that does not have a GPS feature.

I have personally experienced a small number of bugs with Subsurface but I found it very easy to report them and they were fixed very quickly. If no one reports them, they won't get fixed.

The interface is not the greatest but the information it provides more than makes up for that. 

One of the best features of Subsurface is its calculation of SAC (Surface Air Consumption).  Note that it actually calculates RMV (Respiratory Minute Volume) which is more useful than SAC but calls it SAC.  True SAC is not all that useful unless you always use exactly the same sized tank.  RMV calculates the the rate at which you consume air normalised for the surface.  In metric, it tells you litres/minute; For non-metric it is cuft/minute.  Knowing your RMV and comparing it between dives is really useful for improving your air consumption.  It should be added that the RMC in Subsurface is super accurate because it uses the actual compressibility of the gas you are using rather than treating it as an ideal gas.

While Garmin Connect/Dive allow you to enter your SAC (not RMV) they don't calculate it for you.

Yes, Subsurface has no web interface, but you can export your dives in divelogs.de format https://en.divelogs.de/

If you are a Windows user, you might consider https://divinglog.de/english/features/features_cloud.php?Site=Desktop which allows you to sync your dives to various online logbooks.

I know this was from years ago, but the Garmin Descent MK2i is a bear to get to work in Subsurface.  Basically I have to download the "Activity" folder when it's connected, then point Subsurface to that folder.  It works, but that just silly IMO.  I'm HUGE fan of Subsurface, but if you follow some of the threads about getting a MK2i to work with Subsurface on Win10, and it's clearly a tough deal and you're lucky if it works at all.  And beyond that (and I just verified this again this morning) the Subsurface mobile App doesn't have an option to upload any Garmin model.  I love my MK2i. and I love the Subsurface App, but sadly they just don't seem to like each other.  It's like me having two BFFs that happen to not like each other...  UGH!