Online orders can be returned as standard via the postal method listed here https://www.decathlon.ie/en/content/8-365-days-return using An Post or Coll8 and this is for a refund only within 365 days of purchase.
The exchange of online orders for different sizes or alternative products is not possible via this method but can be done in store where a refund can also be chosen if a suitable alternative is not available.
You would just need to bring a copy of your orders invoice with you to the store with the unused/unworn item and the in store team will be able process a refund or exchange if chosen following inspection.
Please note all returns are subject to inspection and may be denied if terms of return are not met.