Method 1: With Blessings
Does NOT apply to GPG original maps
Contact original author and obtain permission to rework a map, or find a map with an open license (e.g. Survivial Genesis, Adaptive Alpine Valley)
Download the latest version of the map
Rename the folders, filenames, and file name references within the files to a new, unique name
Open with editor of choice, and make changes you desire
Upload and enjoy
Method 2: Without Blessings
NOT ethically sound
Download the latest version of a map
Open two map editors simultaneously on the same screen
Load the map you want to remix on one instance of the editor, and make a new map on the other
Make a map from scratch, using the existing map as reference
This will get you in trouble if you cannot reasonably prove you made a map from scratch and didn't directly copy any existing features