Hey,
To change Upgrade and Update billing details button, you need to do the changes in template file.
For Update billing, you need to do changes in file mentioned here near line no 17 :
root\plugins\payplans\stripe\app\tmpl\button.php
<a href="javascript:void(0);" class="btn btn-pp-default-o" data-stripe-update-<?php echo $uid;?>>
Replace with
<a href="javascript:void(0);" class="btn btn-pp-primary" data-stripe-update-<?php echo $uid;?>>
For upgrade button, do the changes near line no. 212 in file path :
root/components\com_payplans\themes\wireframe\dashboard\default\default.php
<button type="button" class="btn btn-pp-default-o" data-upgrade-button data-key="<?php echo $subscription->order->getKey(); ?>">
<?php echo JText::_('COM_PP__APP_UPGRADE_BUTTON'); ?>
</button>
Replace with
<button type="button" class="btn btn-pp-primary" data-upgrade-button data-key="<?php echo $subscription->order->getKey(); ?>">
<?php echo JText::_('COM_PP__APP_UPGRADE_BUTTON'); ?>
</button>
Let me know if you have any query.