For starters on Web 8, the XPM button is on the top right, and the CME URL in your pages is coming from a configuration file, which is generated when you publish the Publish Settings Page in the _System Structure Group.
When you are using Web 8 Topology Manager (the default setup), this URL will be read from what is in your Topology Manager under Content Manager Environment configuration. If that is incorrect, you should change it. After it has changed, you need to republish the Publish Settings Page in the _System Structure Group, and then call the /admin/refresh URL on your DXA site, to clear the cache of the web app configuration.
If you use 2013 SP1 or legacy Publishing on Web 8, DXA gets its XPM CMS URL from the Environment Configuration Component in 100 Master\Building Blocks\Settings\Core\Admin. It can never harm to make sure that URL is also correct, and again, after updating it, republish the Publish Settings Page in the _System Structure Group, and then call the /admin/refresh URL on your DXA site.