How to change a user account?

Though You have not mentioned which version of an operating system you are using, i am taking into consideration that you are using a version of Windows, earlier than the Windows 7.

In case you have forgotten passwords for any administrative account, here`s something you can try.

When you are booting your computer, preferably a cold booting- that is starting up from a switched off mode, keep pressing "F8" button continuously. When a black screen comes asking you whether you want to run windows in safe mode along with some other options, Choose to start windows in safe mode.

When the booting is complete and windows has opened in safe mode, click on control panel from the start menu. Then go to user accounts. There you can delete all unused accounts, and choose to create a new user account with administrative rights.

Then restart your computer in normal mode. and log-in to newly created account.

I hope this will solve your current problem.