Here we cover how to cleanly merge one term into another. This is a two part process that will move the term along with the products and update your system.

1. Once you’ve decided which terms you want to merge, in your admin menu go through “Structure” -> “Taxonomy Manager” -> “Category”. This will take you to your category tree.

    a. Here you will select the term you want to merge into another. In other words, pick the future child term first. You can look through the category tree by expanding terms with the “+” box to the left of a term, or my searching for the term:

        

    b. After selecting the new child term, click on “Term merge” from the options in the toolbar. In the new dropdown frame, start typing the name of the new parent term to the one you already selected. When you see it pop up as an option, click on it and then click “Merge” below.

    c. When this finishes the page will refresh and you’ll notice the change is made. Now it’s time to update the rest of your system.

2. Now for the second part, in your admin menu go through “Store”-> “Products” -> “Term Parents”.

    

3. At this point, you will need to reindex your site. It's just a click of the right button. In the admin menu go through "Configuration" -> "Search and metadata" -> click "Search API", then click "Product Display Search Index", and finally click "Queue all items for reindexing", and "Index now". This will update the rest of your system.