I have figured out the problem. First let me tell you, there are two types of solution.
1st: To delete any account(FB, Google etc), go to your phone settings. You will see "Accounts" option. Enter. Click on the type of your account(Google, Facebook).
To remove the account , click the 3 horizontal lines icon on your phone's top right. You will see 2 options. i) sync account, ii) remove account.
You know what to do.
Now for the 2nd solution
2nd: In this case, you will not see the 3 horizontal lines icon. Therefore you won't be able delete your account like I told you in 1st solution. Instead after you click your account you will have to press the 3 horizontal lines in your phone. Don't get it ? Check the bottom of your phone. Not the screen. You should have 3 buttons. There you have the magic button.
You will see the same options. Sync account and remove account. Again you know what to do..
Thanks for seeing the answer. I hope it helped.