Hello,

how do I modify the login form? I want to put a login button, which will call a pop-up window with login and password fields. Also, I want to change the registration form, where I need only two fields to fill.

How do I do that?

Hello,

First of all you will have to create template overrides for any modification to the files. Read here getk2.org/documentation/tutorials/174-templating-with-k2-and-the-concepts-of-sub-templates how to do so if you are not already using overrides.

If the login form is loading through K2 User module then you should create an override for this module and make the modification needed in login.php file.
The registration page is loading through register.php in com_k2 template files.