I just accidentally put the track title in the track number field. Is there a way to delete everything in that field for all files? I had a look around and couldn't work out whether it's possible or not. Thanks.

Select all files in the files list.
In the tag panel: for TRACk select <Remove>
Click Save in the toolbar. (or Ctrl-S).

If there are a lot of files then this may take a little time.

Please note: if you have added the title to the data already in TRACK then it may be possible to just cut off the title and so keep the track number.
But without example it is very hard to explain from the distance.

In the drop down lists of the Tag Panel it's "<blank>".

To the OP: Anywhere in Mp3tag, when a field is set to <blank> or a value is set to an empty string, Mp3tag will remove the field rather than write a field with any empty value.

What you may want to do, though, instead of completely removing the TRACK field, is use the auto-numbering wizard to renumber the tracks.