Can I change the trade or business type after creating the company?

No — the trade is fixed for the life of that company, because it decides which fields and features (batches, expiry, weight-based pricing, table billing) are already built into your bills, stock and reports. Switching it would leave old records in a shape the new trade cannot read. If you genuinely picked the wrong trade, the honest fix is a new company with the right one — Settings → All companies → + Add company.