Since Electrum set opt-in RBF 'true' by default, you can simply right-click on it and select "Cancel (Double-spend)".
It will create a new transaction that'll send the BTC back to yourself, set a higher fee, sign and broadcast it.
That's unless you're using an old version.

If you do not want an extra transaction that will send it back to yourself, do o_e_l_e_o's method to send your next payment (to others).
To expand the part with "...spending those coins": make sure to select the exact UTXO that it tried to spend in the 'Coins' tab (View->Show Coins) by using coin control (instructions).
If you have multiple UTXOs in the coins tab, Electrum might not auto-select the UTXO that you need to spend.