Hello and welcome @santiyounger 
You can change any site text, including plugins, from the /admin/customize/site_texts page. If you copy a snippet from what you want to change and paste it into the search box there, it should bring up the right fields to edit. 
In this case, the “Support Us” one is js.discourse_subscriptions.campaign.title, and the “We need your help to keep this community up and running!” is js.discourse_subscriptions.campaign.body