Originally Posted by KingCanute

If you book a cash economy or business fare you can use them to upgrade into business or first respectively (if there is first on your route). But it only works if there is already standard rate avios availability for a redemption in the higher cabin, so it's worth checking that before booking. I haven't seen the option to upgrade with Qcredits during the booking process. You need to go back into the booking or use the upgrade link from your member dashboard once ticketed. If there is redemption availability as descibed you might also book over the phone and then ask the agent to process the upgrade straight afterwards to save time

Otherwise you can use them for extra baggage, change or award fees, or lounge passes

https://www.qatarairways.com/en/Priv.../qcredits.html

If you don't use them before the expiry date they are gone. But I believe you can apply them to a flight after that date, provided they are redeemed before expiry.

Bear in mind that Qcredits will be changing to Keys (possibly imminently), likely with different uses and/or conditions.