Like everything else on the Interweb, read this completely before you attempt it. While this worked for me, there is no assurance that it will work for you or your technology. You perform these actions at your own risk, so if you brick your device it's on you.
I just recently upgraded the GPS in my 2015 Sunseeker. It is the Jensen VM9726BTC. I found several problems with the upgrade process. Here is how I worked around the issues.
The updated map set is too large for the micro SD card that came with the unit. I had to purchase a larger micro SD card. Here is what I purchased: SanDisk Mobile Ultra microSDHC 16GB UHS-I Memory Card - Walmart.com
Now we begin ...
1) Download and install the Naviextras Toolbox software.
2) Make a backup of your original microSD card before performing any of the following steps. This backup will be used later. Shut down the naviextras tool box and remove the original microSD card from the computer and place it in a safe place.
3) Insert the new larger microSD card into the computer. Start the naviextras tool box. Use the restore feature to copy the contents of the original microSD card to the new microSD card.
4) When the restore was complete I received the message that the unit had not been shut down correctly. I basically ignored the message and continued with the upgrade process. While it will probably work ok for you too, you do this at your own risk.
5) Once you have "recovered" your GPS data to the new larger card you may proceed with the upgrade using the new larger card.
6) THIS UPGRADE MAY TAKE A LONG TIME! My Internet speed is about 1.5mbps. It took over 8 (yep 8) hours to complete the download of the new data files.
8)Once the upgrade is complete, make a backup of your new microSD GPS card.
9) Plug in the new card and take it for a test drive. I found an error in the map data within 5 minutes, but most other locations have worked well.
We're taking a trip this weekend. I have multiple backups in place just in case this Jensen unit leaves me wandering in circles. (My trusty Garmin unit, iPhone, and Android tablet with GPS enabled. I have this thing called a map, too.)
Hope this helps.