Electrum
To create a transaction that can be replaceable, go to Tools > Preferences and check the box "Enable Replace-By-Fee". Then when you want to send some Bitcoin, next to the Fee slider is a box labeled "Replaceable". Check that box so that while the transaction is still unconfirmed you can replace the transaction with one that pays a higher fee.
To increase the fee of a transaction that uses Opt-In RBF, right click the transaction in the history list and choose the "Increase Fee" option.
This content still holds true for a older version of Electrum wallet and it might be needed by Users of older versions of Electrum.
However, the new Version now supports an
Automatic RBF inclusion for every transaction made so there's always a chance to replace or simply Bump transaction fee if the need arises for all Electrum Wallet Users