My Lumia 520 have been make demo content device nd make many problems eg-not sync microsoft account,

[Happy Rdv]

My Lumia 520 have been make demo content device nd make many problems eg-not sync microsoft account,not add other account like google,yahoo etc,nd not reset phone.

 

[Skamath]

My Lumia 520 have been make demo content device nd make many problems eg-not sync microsoft account,not add other account like google,yahoo etc,nd not reset phone.

I gather you have a demo model.
I tried posting the link here but It gets marked as spam mysteriously. Please google how to flash stock ROM and follow links from good sites like WPC or XDA developers. Hope it helps

Cheers
S