Change SQL Server service account without restarting SQL Server
...for security reasons we want to change the password that starts the sql server service everyday


I realize this is probably outside your control, but it's worth mentioning that this is not a good idea for security.  It's a high process overhead for your organization (even if it's generally automated in some way), and it will likely lead to increased chances of some kind of password breach.  

Think about it like this: every time you open the bank vault, there is a risk that some malicious person can get inside.  Would you rather open the vault every single day and change the locks, or open it occasionally and keep a really big lock on it (long, secure password).

Accessing the password every day (to change it, and then apply it to the service account) is like this.  You should very a very secure password over changing the password frequently.



If you are required to do this, though - why don't you want to use SQL Server Configuration Manager?  That is the recommended method per the docs:

SQL Server Configuration Manager - Changing the Accounts Used by the Services


  Always use SQL Server tools such as SQL Server Configuration Manager to change the account used by the SQL Server or SQL Server Agent services, or to change the password for the account. In addition to changing the account name, SQL Server Configuration Manager performs additional configuration such as setting permissions in the Windows Registry so that the new account can read the SQL Server settings. Other tools such as the Windows Services Control Manager can change the account name but do not change associated settings. If the service cannot access the SQL Server portion of the registry the service may not start properly.


Emphasis added by me.  So doing this outside of the service can cause reliability issues with your database engine.

Regarding avoiding a restart, using the configuration manager accomplishes this goal as well:


  As an additional benefit, passwords changed using SQL Server Configuration Manager, SMO, or WMI take affect immediately without restarting the service.


Of course, as other answers have mentioned, you should be allowed to restart the service from time to time (Windows Updates, etc).