How to edit something in a plugin

Hi Guys,

I want to edit a small thing in a plugin but dont know how to do that. It not CSS but in the plugin itself how do i need to change this in my childstheme?

Its about the: https://www.dreamfysiq.nl/members page and then at the INLOGGEN button the text part “wachtwoord vergeten KLIK HIER OM TE RESETTEN” needs to be aligned more to the left and gets some margins like this:

#wpmem_login .link-text {
width: 60%;
margin: 8px;

But i dont know where to edit this and keep it this way when a update will be done. I know how to add a CSS sheet to my childstheme but how to do that with a plugin?

Many Thanx!