This add-on helps you reduce the number of decks in your collection, by moving cards from child decks into their parents.
Please start by using File>Export to create a backup of your collection with scheduling information but without media. Then after restarting Anki, you should find a "Merge Child Decks" option in the Tools menu. You will have a chance to confirm the changes before they are made. After making the changes, please check that things in the Browse screen are as you expect. If you do not like the new layout, you can restore from the backup you created and try again.
If a field called exactly 'deckmerge' is present on the notes, the original deck name (including the child part) will be placed into that field.