First, hover over settings and then click tags.
Now let's say that you wanted to take two tags and merge them together into another tag. It's really simple.
Start by going to the tag you wish to combine and click on the check box to select all subscribers and then use the bulk actions feature and add contacts to the existing tag.
If you want to verify that you’ve done this correctly, click on tags and you will see that the number has increased. You can do this with as many tags as you want if your goal is to merge them together.