Update Role

Hello,
First of all, i would like to congratulate you for your amazing plugin. It is a big offer to the wordpress community.
So, I have created a custom template, and i want to change the role of my users when they click a button in the website and gain access to new content.
To do that i want to change-update or whatever their roles with a new role that i have created.
I have tried set_role(), update_role() and other similar functions but with no success.

When i use the following:

um_fetch_user( get_current_user_id() );
$member_role = um_user(‘role’);

i am able to retrieve the current user’s id and the role but i can’t seem to find a way to set the new custom role that i want to assign to him.

Could you please give me some directions on how to resolve this issue?

Thank you in advance.