Use the Convert Tag -> Filename
Format string:
%TITLE%
This copy the content of your TITLE tag and overwrite your current filename.
You see immediately what new filename you would get before you press the OK Button.
Please be aware that every information in your filename that is not already also available in your various tags (like TITLE, ARTIST, ALBUM, TRACK...) will be lost and overwritten with the content of TITLE only.
For vice versa:
Use the Convert Filename -> Tag.
The format string highly depends on what your current filename contains. If you let us see a real existing filename, we can suggest a working format string like
%TRACK% - %ARTIST% - %TITLE%
if the track would look like
01 - Michael Jackson - Billy Jean.mp3