If you have a tag you'd like to move into an existing Tag Folder you have two options.
Find your Tag
Navigate to "Settings" > "Tags" and all of your available tags AND folders will now be listed. Scroll to the tag you would like to move and select "Move". A prompt will appear with a drop-down list, allowing you to choose a folder in which to place that particular tag.
Find your Folder
Navigate to "Settings" > "Tags" and scroll to the Tag Folder into which you'd like to move your tag and click "Edit". A prompt will appear with two columns allowing you to choose from available tags in the left column by clicking their names, which adds them to the folder in the right column. You can also click the trash icon next to tags already in a folder to remove them (see video above).
Note: To remove a tag from a folder, navigate to the folder. select "Edit" on that folder, then select the tag you want to remove or rename and select the delete option to the right of the tag, this will only remove the tag from the folder it will not delete the tag permanently.