Ok here is how to Swap Terrains in the Mission Editor.
First you open the .mis file of a mission, in this case we take the last mission of Frozen Friendship which is c3m8.mis.
Then you look for the lines marked in red!
Then you open the editor.exe with a Hexeditor and look for the terrains .pcx files.
Then you override the lines in the Hexeditor with the ones in the .mis file.
The first three .pcx files are the most important. If you are swapping a snow map for another snow map you don't need the last three
You have to be careful that the number of letters match the ones in the hexeditor.
This is the new map added to the mission editor.