problem with device acount.

hi. i dont know where to post this.
but i have a smallproblme on my device.

i have a free acount to install app on my iphone
but a friend give me (gift) a reddeem code to install one app.
and the iphone device change my acount to the acount of my friend.

now the iphone looks like the owner is my friend, jaaj
now every time i want to install a new free app, askme for the password of my friend.

question:
how can I return to my old acount on my iphone device plese.?

thansk a lot