You can, and there are two ways depending on your user config:
1) if you are using Anonymous users you can configure the login page in your modeler as a role-specific homepage for anonymous users (See https://docs.mendix.com/howto/ux/setting-up-the-navigation-structure)
2) Without Anonymous users enabled, navigate to your Mendix project folder (usually Documents/Mendix/<Project Name>) and within your theme folder you should have or can create an login.html page which users will go through to log in.
Hope this helps.
Thanks for the quick update.
If we go with the 2nd option then how to customize that login page. Can we edit that login page manually?
Or is there any way to customize that login page from Mendix ?
I want to add a sign-up functionality on the default login page and I am unable to.