Can you take back your bid?

No: once your bid is spoken and heard, it is firm and final. You can't correct it.

A bid commits you

During the auction, the moment you call a value and a suit (say "90 hearts"), you're committed. The classic rule is that a clear bid heard by everyone can no longer be changed: no "actually, make it 100."

The slip-of-the-tongue case

If you misspeak (a genuine slip corrected at once, before others have reacted), some tables tolerate the fix. But it's a courtesy, not a right: in a tournament, the bid as spoken stands. Best to think before you speak.

Raising isn't changing

You can always raise your bid on your next turn if the auction comes back to you (for example going from 90 to 110), that's normal play. What's forbidden is taking back or lowering a bid already made. To bid is to commit. Likewise, once you've passed, you can't speak up again to take the bid: passing is final for the current round.

See also

FAQ

Can you change your bid in coinche?

No. A bid clearly spoken and heard is firm and final. You can neither lower it nor correct it afterwards.

What if you misspeak when bidding?

Some tables tolerate an immediate correction of a genuine slip, but it's a courtesy, not a right. In a tournament, the spoken bid stands.