TNUoS charges are set using your site’s registered connection capacity, measured in kVA. That’s the capacity recorded by the network, not your day-to-day usage.
If that number is wrong - for example, because your site was over-specified years ago or your operations have changed - it’s worth checking. We can confirm what capacity is currently on record and sense-check it against how your site actually runs.
If the recorded capacity is correct, reducing it isn’t something a supplier can change unilaterally. Any formal reduction has to go through your local Distribution Network Operator (DNO).