I would like to rename multi-file-applications to multi-file-programming. It sounds better and is not necessarily specific to applications. multi-file-applications also sounds a bit weird. How can I rename the tag?
How to rename a tag in general is explained at the link posted by @BartvanIngenSchenau.
But let me add what I think about this specific case. The tag multi-file-applications was only used for two questions on this site, both are closed for now. One of the two questions had severe quality issues, so I am astonished it was not deleted in between. The second one of the two questions was asked by you, it was highly upvoted and got a good answer (as a side note, it was also closed, which is perfectly justfied according to the site rules, we talked about it).
But don't forget, your question got a comment by @TannerSwett which was currently upvoted 73 times:
By "multi-file programming," do you mean writing software using more than one source code file? That's not really called "multi-file programming," for much the same reason that a brush that has multiple bristles is not called a "multi-bristle brush."
And now you want your former question to be tagged with exactly this - "multi-file-programming"? I think the usefulness of this step is quite debatable.
If you still feel your question should be tagged differently, then simply delete the old tag from it and add the new one - you already have the 300 reputation points which are required for creating new tags.