If you have previously installed Excerpts+ on your site, you will need to follow the instructions below, for Installing on a site with Excerpts+ already installed.
Headway blocks are plugins, and as such, are installed using the WordPress plugin installer. For this, you will need the Excerpts+ ZIP file that you downloaded from Headway Themes. On some systems, it may have been automatically decompressed. In that case, check in your computer’s Trash or Recycle Bin and copy it to somewhere on your hard drive.
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The quick version
Use your site’s WordPress plugin installer and activate the plugin once installed.
Step-by-step

In your WordPress, under the Plugins section, select the ‘Add New’ plugin option and upload the Excerpts+ ZIP file.

Activate when prompted
That’s it! You can now move on to chapter 3, on Using Excerpts+.