Bonds are renewed and reinvested automatically unless you tell NS&I otherwise.
This means you need to decide if you want to cash in all or part of the bond before the maturity date of the bond. NS&I will write to you around 30 days before your bond matures to explain your options in detail. You don’t need to do anything until you receive your pack.
Once the bond has been renewed there’s a penalty for withdrawing that’s currently the equivalent of 90 days’ interest.
If you decide to renew, be aware that the interest rate could be higher or lower than the interest rate on your existing bond.