a) Load all tracks into Mp3tag
b) Select all tracks
c) Convert -> Filename-Tag -> Format string: %title% -> OK
If you only want to copy a part of the file name into the TITLE field, you will need to show us the exact file name and specify what you want transferred.
For example, if you don't want to copy the leading tracknumber and the space in 01 filename, you could use as format string: %dummy% %title%
a) Use Convert Filename -> Tag with the format string %title%
(Only use this way after you have successfully changed your filename.
This will "copy" your complete filename into the TITLE tag.)
or
b) Use Convert Tag -> Tag with the formatstring $num(%_counter%,2)_%title%
if you only want to add an incremented number and an underline in front of your existing TITLE content. In this version, the content of TITLE will not automatically match the filename!
Could you please tell us exactly what you do?
Also, it would be nice to see which function you used, if you got any error messages what you expected, what the underlying data is.
We cannot look over your shoulder.
Because @LyricsLover 's suggestion works fine over here:
Yes, I know. What I don't know: what you have tried so far. As you see from @LyricsLover 's and my suggestion: there is a way that works. And if it does not, you have to show us what you do.