The two "do-it-yourself" ways would be

• to try to issue abandontransaction "txid" in the command console of your core.  I've had limited success with that.
• restart core with "-zapwallettxes=2, which will give you back control of the unconfirmed funds - just know that the unconfirmed transaction still exists.  The goal now is to usurp the unconfirmed transaction and respend the same funds with adequate transaction fees.  The "second" spending of the funds will get confirmed, causing the network to mark your original transaction as an invalid double-spend.
  

More details can be acquired by googling the core commands given above.  If you're adventurous, add "rawtx" and "sendrawtransaction" to the search terms for the do-it-yourself transaction accelerator.