If you need an atomic modification, unlinking won’t work.
A better solution would be to create a new temporary symlink, and then rename it over the existing one:
os.symlink(target, tmpLink)
os.rename(tmpLink, linkName)
You can check to make sure it was updated correctly too:
if os.path.realpath(linkName) == target:
# Symlink was updated
According to the documentation for os.rename though, there may be no way to atomically change a symlink in Windows. In that case, you would just delete and re-create.