How to change tag version

I want to change Flac(Flac ID3v2) to Flac(Flac) to make my ak100 able to read it

up

Select the files, press Ctrl+X to cut all tags, then press Ctrl+V to paste the tags back.
Id3v2 will not be written to flac files so they're removed.

You might want to back up your files or just the tags before the operation because once you've cut the tags they're only saved in the clipboard until you paste them.