I suppose you would have to first copy the hard linked file, then delete the file that was the hard link.
So, if you had:
file1.txt
& you created a link to that
file2.txt
file1 & file2 are now one in the same.
Make a copy of file2, 'file2 - Copy.txt'.
You now have:
file1.txt
file2.txt
file2 - Copy.txt
At that point, file1.txt & file2.txt are one in the same (still hard linked), & 'file2 - Copy.txt' is it's own file (though it happens to be a copy of them).
Now that you have that individual file, 'file2 - Copy.txt', delete file2.txt.
With that, you'll then have:
file1.txt
file2 - Copy.txt
They are both individual files, not linked in any manner.
(Their contents happen to be the same, but they are not the same file, are not hard linked.)
---
Experiment.
Make some hard linked files to test with, & understand what that entails.
Maybe go back to file1 & file2...
so file1 (hard linked to file2) contains the word, "hi".
since it is a hard link to file2, file2 also says "hi". (they are one in the same)
create a copy of file2, 'file2 - Copy'.
at that point, file2 - Copy' also contains the word, "hi".
all 3 files contain identical contents, yet file1 & file2 are one in the same, 'file2 - Copy' is its own individual file - not linked to anything else.
now edit file1, changing the word "hi" to "bye".
with that...
file1 says, "bye".
file2, a hardlink of file1 says, "bye"
file2 - Copy, its' own individual file, says "hi"
at that point, delete file2
file1 & file2, previously linked, the file2 reference now "unlinked"
with that, you now have...
file1, that says "bye"
& 'file2 - Copy', that says "hi"