ANHAD

Could anyone please enlighten me when does Anhad turns to Gurbani , thanks,

It doesn't.

Firstly, you want to be sure that you know what "anhad" is. This can only be understand from people with experience.
Secondly, it is "via" the anhad that one gets experiences. These experiences vary according to avastha and inner cleansing.

(Firstly, you want to be sure that you know what "anhad" is) ? Can you please explain than, thanks.

The word Anhad has been used as an adjective and NOT as a noun. Here are a few examples:

ਅਨਹਦ ਬਾਣੀ ਪਾਈਐ ਤਹ ਹਉਮੈ ਹੋਇ ਬਿਨਾਸੁ ॥ (21)
ਅੰਮ੍ਰਿਤੁ ਵਰਖੈ ਅਨਹਦ ਬਾਣੀ ॥ ਮਨ ਤਨ ਅੰਤਰਿ ਸਾਂਤਿ ਸਮਾਣੀ ॥ (105)
ਪ੍ਰਭ ਕੈ ਸਿਮਰਨਿ ਅਨਹਦ ਝੁਨਕਾਰ (263)
ਵਾਜੇ ਤਾ ਕੈ ਅਨਹਦ ਤੂਰਾ ॥ (393)

So the word anhad is describing a quality of the noun ਬਾਣੀ (gurbani or waheguru naam), ਝੁਨਕਾਰ (music of baani or naam), ਤੂਰਾ (divine musical instruments sounding in sift salah ).

ਅਨਹਦ is a another form of the adjective ਅਨਹਤ which is used to describe a sound which is produced without a strike against an instrument; a divine sound, a celestial sound.

Therefore Anhad is not a thing in itself.
Gurbani can be said to be anhad, Naam can be said to be anhad, the divine experience of these in the minds of Gurmukhs can be referred to be as anhad sounds or anhad experience. But Anhad is not a thing in itself.