Please note that you won't be able to increase the fee if you use old version of Electrum. RBF (replace-by-fee) is turned on by default since the Electrum 3.1.0 which was released somewhere in March. In such case, updating Electrum won't help because the transaction has been already broadcasted.
According to the documentation on the Electrum FAQ you would have needed to checked “Replaceable” on the send tab before sending the transaction. If this is enabled by default now (I can't check right now but I've it is before) then you just create a child pays for payment which is using a different address to pay for the fee of another address. This would require you to have some left over balance obviously.
To create a CPFP transaction just right click on the unconfirmed transaction that is in your history tab and choose “Child pays for parent”. Set a fee which will get it confirmed in the time span you want. There's many places you can check the current network fees and get a good idea how long it will take to confirm. Then click on “ok”. A window will popup and you just have to click on "sign" and then "broadcast"
This should then get your existing transaction to use the child pays for payment fee and get it confirmed quicker for you.
You don't need to check replaceable to do CPFP. CPFP only requires that you control one of the output addresses. RBF, which is another way to bump the fee, requires that you mark a tx replaceable. Yes the default is for all transactions to be marked replaceable.
