Assuming you have a whole bunch of files that you need to change their extension from one to another, you can use the following commands after setting the values for the BEFORE and the AFTER variables to the values you need.
BEFORE='.txt'; AFTER='.csv'; find . -type f -name "*$BEFORE" -exec bash -c 'mv "$1" "${1%$2}$3"' _ '{}' "$BEFORE" "$AFTER" \;
What the above will do is: after setting the two input variables it will call find in the current directory (with recursion) and find all files that their suffix is the value you set in BEFORE. Then, for each match it will create a new shell terminal in which it will rename the file by removing the old suffix and then attaching the new one. We pass the input variables as parameters to the new shell and that is why inside the code of the shell we are using variables $1, $2 and $3. The reason we had to issue a new shell is because we wanted to reuse the ‘{}’ variable.