Can I cancel or lower my bid?

まだ日本語では提供されていないため、原文で表示しています。上のメニューから翻訳できます。

No, not while it is the highest bid. Bids are binding, and the winning bid pays for the item when the auction ends. Once someone outbids you, you can withdraw your bid as described above.

Check the amount before you confirm. The bid screen shows the exact figure you are committing.