Hope this helps anyone with the same issue:
UPDATE r SET HardwareNumber = r.NextID FROM (select 999999899 + ROW NUMBER() over (order by HardwareNumber) AS NextID,HardwareNumber FROM tblAsset WHERE HardwareNumber is NULL) r
UPDATE r
SET HardwareNumber = r.NextID
FROM (
SELECT
999999899 + ROW NUMBER() over (order by HardwareNumber) AS NextID,
HardwareNumber
FROM tblAsset
WHERE HardwareNumber is NULL
) r;