I have company arriving today, but I will have time to detail instructions tonight. Meanwhile, go to the following link and download and install "GPS Utility" file version 5.50 and read the instructions. It is a free utility but you may like to make a donation to the creator.
In a nutshell, you install the micro SD device into the chart plotter and format it. Then save all of the existing waypoints, and routes, and tracks to it. Then put the micro SD card into an adapter and then into the computer and save the file to the computer. It will be in a particular folder on the SD card and it will have an extension of .adm.
Then, using the GPS Utility, input the .gps file with all of the new waypoints & routes and then save it as a .adm format. Copy that adm file to the SD card and then load into the chartplotter.
I can answer questions tonight if you have any.
Let me know how you make out.