Start with the Microsoft account order
The Taiwan Xbox listing identifies GTA VI as an Xbox Series X|S preorder. Match the product and edition in your order with the listing for the purchase you intended.
Microsoft's cancellation guide links to Order history. Sign in with the account that placed the preorder and locate the GTA 6 transaction before choosing what to do next.
Sources: [1][2]
Cancel when Cancel item is available
Microsoft's order guidance says to open Order history, select Cancel item next to the order or preorder, select the item, and confirm the cancellation. Check the inbox for Microsoft's confirmation email.
If the confirmation email does not arrive, check spam or junk mail. If it is still missing after waiting, use the Microsoft Store support link in the cancellation guide.
Sources: [2]
Use the Xbox refund route after billing
Microsoft's Xbox-specific cancellation guidance directs you to the Xbox refund webform when the Cancel item option is unavailable after billing. The Taiwan refund link below is for Taiwan; use Xbox Support for your own account region if it differs.
Open the refund route and follow the current instructions for your transaction. Microsoft's apps-and-games support page separately identifies this as the refund route for Xbox digital purchases.
Sources: [2][3][4]
What to keep before contacting support
Keep the order number, the Microsoft account used, the storefront region, the product name and any cancellation or billing email. Keep passwords, recovery codes and complete payment-card numbers private; never include them in public comments.
Use the store named on your receipt to identify which support service to contact about the purchase. Keep the order reference with any follow-up so support can identify the same transaction.
Sources: [2][3]