Perhaps you mean "how do I claim a warranty repair?" Visit an Apple store, or an AASP. If the iPhone was puchased in a different country, it must be returned to that country for a warranty repair. Accidental damage is not covered by the warranty. For a non-warranty repair or replacement, Apple charges approximately $300, in the US. If you live outside of the US, the charge may differ.
Apol's Carl - off topic, but you're knowledgable so probably will know this answer......
I'm looking to buy a new 4S here in Cambodia [as my near new 4S was stolen a few days ago and I'm still travelling for another 52 days].
The retailers here say - No Guarentee! I assume they mean - no warranty
Upon my return home to Australia, if any issues arise [sticky home key for example], I'd just need to pay. There'd be no warranty from Apple for 1 year. Correct?