Edit menu item title

Hello,

for my new plugin I want to change a menu item title and a page title via code. I am developing a customized login-function and while the user is logged-in the menu item title should change to logout and back after the logout. The same should work with the page title, but there is another possibility: Hide title and create in each case the correct headline in plugin.
How can I change the menu item title?

Thanks

Ottilie