You can withdraw part of your stake as long as three days have passed since you last staked in, and provided that the minimum staking amount is maintained after withdrawal.
Withdrawing the stake might lower your chance of being selected as a validator in the days after, depending on how much others have staked.
Go to Validator Dashboard > My Staking > expand the options > Withdraw.
Enter the amount manually or choose Max to withdraw the entire stake, and then select Withdraw.
Confirm the transaction.
After the transaction is confirmed, you can review its details on the Activities tab or check the status in Ronin Block Explorer.