I have to change the user from my dads name to my moms. So the sender should be my mom from now on.
How can I change the user ?
Go to menu/tools/accounts and in the general tab, you can change the user name.
In most cases that will work, but not all.
Some email providers, like outlook.com, set the name on their end, so you would have to go to the website and change the settings there.