Sigh...
Why did you change extension for "project.c3proj" file?? Rename it back from "project.c3p" to "project.c3proj"
Go back to "bamboo forest.c3p" file. Rename it to "bamboo forest.zip"
Extract files from "bamboo forest.zip" into an empty folder.
Open "project.c3proj" file in Notepad++, make changes, save.
Don't do anything else. Try opening this file in Construct 3 editor.
If successful, make a backup copy of the entire project.