based on what you want choose one of the following
0. backup your map
1. open the world editor
2. click on "Advanced" in the main window
3. click on "Replace Cliff Type"
1. open the world editor
2. click on "Advanced" in the main window
3. click on "Modify Tileset"
4. choose another Base Tileset
this might mess up your other terrain so make sure you modify your tileset so that the right tiles are replaced
Then find the cliff what kind of you want to replace old one.
save it.
Import it in world editor.
Find cliff with you want to be replaced.
Open imported file properties, check use custom patch, than clear window with text "use custom path..." and write your cliff texturess adress like: "Replace textures\Cliff\xxx.blp".
Than save your map, restart world editor.
That doesn't work on cliff tiles.