Can't modify, need to cancel the fixed investment and re-sign.

You can enter personal online banking and select Online Securities-Online Fund-Fund Trading-Fund Fixed Investment Management Cancel. After the fund management company confirms (T+2), the fixed investment plan stops and the system stops deducting money. On the day of application, investors cannot apply to withdraw from the fixed investment subscription business of the Fund. After canceling the fixed investment agreement, the purchased fund will not be redeemed automatically.

Or you can sign a fixed investment agreement.