The correct way to do this, is to create a child theme:
https://codex.www.ads-software.com/Child_Themes
And modify the footer.php to remove the credit links in the file rather then just hiding it with CSS (the content still will show in the source code if you hide with CSS, and will still be available to screen readers and search engines for example), creating a child theme for your modifications will mean you won’t have any worries if you need to update your parent theme.
If you were to follow Calvin suggestion, I would not edit the themes style.css
Instead, you might find using a custom CSS plugin a far simpler solution. If you are using the JetPack Plugin it has a custom CSS module alternatively you can look at https://www.ads-software.com/plugins/simple-custom-css/
Apart from making it easy to make CSS changes in your admin area, it will also mean that if you update your theme if a new version is released, you won’t lose your changes.