You can withdraw remaining rewards from a boost either through the Boost UI or programmatically using the SDK.

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Using Boost UI

You can withdraw the remaining tokens from your boost by visiting your budget account dashboard on the Boost App. Navigate to the “Boosts” tab to see a list of boosts you have deployed. Once rewards have been withdrawn, the Boost will be considered ended/inactive.
Boost Withdrawal UI

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Using the Boost SDK

To withdraw programmatically, use the clawbackFromTarget method on the budget.
For ERC20Incentive, the amount specified in the clawback data must be an exact multiple of the reward value set in the incentive. Otherwise, the transaction will fail.
Depending on the type of incentive, calculating the clawback amount will be different. 
// Example for fixed-reward ERC20 incentives
const boost = await core.getBoost(0n)
const budget = boost.budget
const incentiveIndex = 0
const incentive = boost.incentives.at(incentiveIndex)

if (incentive instanceof ERC20Incentive) {
  // Calculate exact amount to clawback based on remaining claims
  const remainingClaims = await incentive.getRemainingClaimPotential()
  const rewardValue = await incentive.reward()
  const clawbackAmount = remainingClaims * rewardValue

  await budget.clawbackFromTarget(
    core.assertValidAddress(),
    incentive.buildClawbackData(clawbackAmount),
    boost.id,
    incentiveIndex
  );
}

SDK Reference

Learn more about the clawback method and its parameters