i have tried this, but it is not working. i need to use the same 'lrm-login' class. So, I am deactivating the plugins..

Please follow the steps below to put the code:

1. Install the Simple Custom CSS and JS plugin: https://wordpress.org/plugins/custom-css-js/

2.  Add the following JS code via the plugin.

public function add_footer_html(){

Or above the ending bracket"}"??

how could I set the class by the second method. I could not understand properly. I have moved the file to my child theme directory. So, I think, there will be no update problem. 

We strongly recommended doesn't use the second method if you don't have a knowledge in programming. This is not the update save way, as Sofy already mentioned.

 and I think it is for the login, so what about the registration??

You just need to use the code provided by Sofy as described here, to make it work:

jQuery(document).ready(function(){
   jQuery('#wpf-msg-box a').replaceWith('<span class="lrm-login">Login</span>');
});

Just change the red marked value, with your class name.

Also please note if you remove the #wpf-msg-box selector from the code above, it'll replace all login links (wpForo). 

For the register page, you just need to change the wpforo=signin to wpforo=signup in the code provided above.

means that another way to change the login link

I understand that. and my question is there. I found the following code in the class-template.php file.

    public function add_footer_html(){
	    $this->dialog();
	    $this->spinner();
	    if( apply_filters( 'wpforo_message_bubble', true ) ) $this->msg_box();
	    if( WPF()->current_object === 'post' ) $this->report_form();
    }

What to add/replace, I could not understand. If I only have to add the below code, above/below which line I should put? and I think it is for the login, so what about the registration?? and last question, do I have to change anything for logout option??

jQuery(document).ready(function(){
    jQuery('#wpf-msg-box a').replaceWith('<span class="lrm-login">Login</span>');
});

1. https://drive.google.com/open?id=1cgHsMj9QXWhKit7Nzu7G-1chEbERcTku

2. https://drive.google.com/open?id=1mJ5AJpckrSK3mciyWplgal_Dhv8By4oe

3. https://drive.google.com/open?id=15ycLSx2xsqDCR6N7SNZtJJ4Gbvu5Uj5J

The second method provided in the above link means that another way to change the login link is customizing the add_footer_html() function and make it work as you need.

Also please provide in detail what you want to change, provide the screenshot, and we will try to help you.