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Here you should find «Change nick» item:
When you click on this item popup window with confirmation will appear and you should click «Yes, purchase it»:
After the purchase click on «Edit profile» in your menu:
You need to click corresponding button in the edit profile form:
Please read warning and write your new nickname in the special field. Check it twice, because we can't cancel results of this procedure:
Scroll down to the end of the form and click «Save». Here your nickname must be changed if everything was done right. Remember that your login will not change so if you change nickname from Bla222 to XXX then your login will still be Bla222.