Can you check my attached screenshot below, is it you referring this subscribe button?

If yes and if you would like to change that word to "Read More", you can override this following language constant from your site backend > extension > language > override

COM_PAYPLANS_PLAN_SUBSCRIBE_BUTTON

You can refer on this documentation how to override this language translation from backend.

https://docs.joomla.org/J3.x:Language_Overrides_in_Joomla

If you want to change some styling, you can refer on these following files.

JoomlaFolder/components/com_payplans/templates/default/plan/default_plan.php
JoomlaFolder/components/com_payplans/templates/tmpl_bs3/plan/default_plan.php (If you using bootstrap 3.x from your Payplans layout (http://take.ms/c5Gqy), you have to modify on this file)

Then move your customisation file into your current template file location JoomlaFolder/templates/YourCurrentTemplate/html/com_payplans/plan/default_plan.php, so it will not affect when you update to next version.

Hope this will help.