Hi there and welcome
What is the make and model of the mobile phone?
You will need to reset the phone, its the way you remove the account as it is all linked to this.
It will remove all data from the phone but you cannot avoid this.
To do the reset, go to Settings, Backup and Reset, Factory Data Reset
Complete this then it will ask you to set up the phone again, then use the new account you wish to use
Let me know the results please
how did it not work for you? where did you get stuck?
ok.. but can you explain to me what you have done so far and what you see on the screen please