Tools > Preferences > Fees > Enable Replace-by-Fee. And tick "Replaceable" when you're sending.
The transactions you sent this way will be marked as replaceable, you can increase the fee by right-clicking and choose "Increase fee". You can mark a transaction as final when increasing the fee, doing this will make you unable to increase the fee again (after you broadcasted the "final" transaction).
Bump, I'd like to see this too.
If you don't mark the transaction as replaceable, there's no way to do it afterwards?
What about just sending a transaction with a higher fee from the same inputs?
1. No way to do it.
2. That's what it does. The fee will be subtracted from one of the outputs. So if you want to increase a fee from 0.1 transaction with 0.1001 input, 0.1 output, and 0.0001 fee by 0.0001, the output will become 0.0999.
Not sure if that's still the case if you still have unused input(s) though.