You normally need to ask for a PIP mandatory reconsideration within one month of the date on your decision letter. Explain which parts of the decision you disagree with, why, and include any relevant evidence; you can usually request it by phone or in writing, following the details on the letter. If you miss the deadline, you may still be able to ask, but you should explain the reason for the delay. Check the latest guidance on GOV.UK, and if the decision is unchanged, you can usually appeal to an independent tribunal within one month of the mandatory reconsideration notice.