First, read the letter
The DWP’s decision letter should tell you which benefit was overpaid, the period, the amount, and why they say it happened (e.g. a change you didn’t report, or their own error). Check it carefully — overpayment decisions are sometimes wrong on the figures or the cause.
Don’t ignore it (that can lead to deductions or debt collection), but don’t just accept it either. Whether — and how much — you have to repay depends on the type of benefit and the cause.