unreal will create intermediary files that will use instead of those you see in the content folder. usually if all goes well when you remove them from the content folder unreal will do its stuff behind scene and update the intermediary files. but this may fail off course.
first save a copy of the entire project as it is. use winrar to backup the entire folder. if something doesn’t work after the following changes you delete the messed up project and you unzip the rar file
then delete all folders specified in this thread. but do not do this if you don’t have a backup