I'm new to CRM CE - and would need your help!

Hello! I'm trying to use the CE version for as a project for school.
I would like to ask how can I add a button either on listView or DetailView that could change the value of a field.

So, I have this module of Quotes, and planning to add a button that could change the Status field from 'not paid' to 'paid'
I've tried searching the community and other sites as well, but everything I found seems cannot cater to my needs.

Also, I have these error or warning showing. I have no idea how I could fix them, because they seem to be working before, but after my computer's restart.
These errors pops up.

SugarCommunity - 1.png

Line Code:

class ViewAddLineItems extends SugarView{

    public function __construct() {

        parent::SugarView();

        $this->options = array(); }

    public function display() {

        $is_group = $_REQUEST['is_group'];

        $is_product = $_REQUEST['is_product'];

        $is_service = $_REQUEST['is_service'];

        $group_row = $_REQUEST['group_row'];

        $product_row = $_REQUEST['product_row'];

        if (!$product_row) {

            $product_row = 0;}

        $this->ss->assign('is_group', $is_group);

        $this->ss->assign('is_product', $is_product);

        $this->ss->assign('is_service', $is_service);

        $this->ss->assign('group_row', $group_row);

        $this->ss->assign('product_row', $product_row);

        $this->ss->display('modules/UA_Quotes/tpls/ViewAddLineItems.tpl');

    }

}

Help is always appreciated! Thank you so much in advance Sugar Community!

PS: I have no Idea what 'In a Place' should I put, I'm sorry in advance.