Find all files with the extension .txt in the entire file system, cut the path to only include the first two directories, replace spaces with new lines, remove all new lines, and then use sed to replace all numbers with an asterisk.
find / -name "*.txt" -print | cut -d"/" -f1,2 | tr ' ' '\n' | tr -d '\n' | xargs sed -i 's/[0-9]*/*/g'