Sion railway station: This is the arrival point that matters. SBB trains from Geneva and Brig run roughly every 30 minutes through the Rhône valley; EuroCity trains on the Geneva–Milan route also call at intervals. The station is about a 10-minute walk from the flat part of the old town, then another 15–25 minutes uphill to Valère or Tourbillon. Do not take a taxi merely to reach the centre.
Geneva Airport: Take the train from Genève-Aéroport toward Brig/Sion; some services are direct, others require a simple change at Genève station. Allow about 2 hours to 2 hours 20 minutes and roughly CHF 50–70 on a standard point-to-point fare. It is decisively better than hiring a car for a one-city stay.
Zurich Airport: Use SBB rail via Zürich HB, then usually Bern/Visp, to Sion. Allow about 2 hours 40 minutes to 3 hours 15 minutes and roughly CHF 80–110 without a Swiss travel discount. Choose Zurich only if the flight is substantially better; Geneva is the natural airport for Sion.