how do i reroll a winner?

If it's the Sakura one, it ended 6 days ago, so only tomorrow you have that option.

You answered your own question? "Please attempt to contact the winner for one week after the giveaway end time before using this category."

update: tried contacting the person that won, they deleted my comment from their steam acc

You have to wait a full week from the time you sent the key, also it's a bad idea to leave a "YOU WON!" comment on steam :I
So, just wait a little bit longer and then hit that Reroll button!

ok, thank you for your help