What to do if you accidentally mark words as known?

If you mark a word as Known, you can't turn it back to a blue word (Unknown Word). However, you can LingQ any terms after making them Known.

Just click to select a term for which you want to change the status back to a LingQ. Then proceed to adjust the status as normal.

Any new words you LingQ will be removed from your Known Words, and your statistics will be updated accordingly.