You cannot reduce the payments made to participants once a study has been published, but these can be increased.
If the reward rate for your study falls below our minimum payment rate, or below the payment rate you set when you launched the study, you will be prompted to offer adjustment payments to participants. This is covered further here: How do I resolve underpaying studies?
If the study payment has not dropped below our minimum payment rate or the payment rate you set for the study, you can still increase participant payments via bonus payments.
We have further extensive guidance on participant payments here: Paying participants.