In the future, please make sure you only mark a giveaway as "Received" after you've activated a game. Providing incorrect feedback can lead to issues with your account.

Whenever you wish to be rerolled as a winner, you will just need to leave a comment on the giveaway to alert the giveaway creator to the situation, like you have already done here. After that it is the creator's turn to either act on the situation or not. If they do not wish to reroll, then you can simply mark a giveaway as "Not Received" after a week has passed from the giveaway's end date. During that week you should leave the giveaway without feedback to prevent any confusion.