Have you discussed this with Dad first?
I am wondering since changing accounts should be so easy, why has Dad not helped you do it? Note that it might change some cookies that mean when Dad then signs on it will be your account until he changes it -unless you clear the cookies. In any case, if you don't have his PW to sign him back in he may notice next time he uses it.
If Dad just wants to keep the browser set to open his account, can you consider adding a second browser - a different one - that only you would use - that could be loaded with your preferences and cookies?
Alernatively, do you have a user separate user account on Windows? This would simplify everything - keeping preferences and files separate. You could both use the same browser - and when it opened it would load the respective bookmarks and cookies for each of you. It would also make it easier for Dad to track your activity and know how you are using the PC.