We removed the click from the Join as it was causing confusion for many. Since these are aligned this way for better view in any device, you'll want to consider the ramifications for small devices if you put them inline. If you only want to change the phrase for "Sign In(or click join" you can do that in the language phrase manager. https://www.socialengine.com/support/article/5308928/se-php-language-manager but your widget looks like the old widget. Here's how it looks on the demo:

Can you let me know what theme and landing page you are using?

You must not be using either of the new landing pages as those both have the new login block I show above. If you don't want to use either of the new landing pages, you'll need to customize the login widget but you'll need to know responsive design in order to make it look good on every device. On that, I cannot help as I'm not a responsive design expert. Have you tried the other landing pages perhaps on a development site to see if maybe one of them would work for your site?  

There are three landing pages with Serenity. Maybe one of the other two would work better for you, https://www.socialengine.com/support/article/98238465/sephp-serenity-theme