How to pratically doublespend bitcoins? I tried to put bitcoins into an p2p exchange with bitcoin-qt. I added a to small transaction fee, so the coins are now stuck as "Unconfirmed". The offer from the exchange is overdue, so all that will happen if the transaction gets confirmed is that I get them back with an administrative fee. 

I would like my coins right now and avoid paying the fee. I've read that you can double spend the coins back to yourself and if I add a higher payment fee than the first time this would probably override the original payment. How can I practically do this? I'm not a hardcore developer but feel comfortable using commandlines and know my way around computers.
Technically, you are not double spending, since the transaction was never confirmed on the blockchain. Double spending is possible by 51% attack; what you are trying is to replace the transaction with one with desired fees, before it is confirmed on chain.

You can use https://github.com/petertodd/replace-by-fee-tools