This accelerator doesn't work unfortunately.
The only one working at the moment is viaBTC (
https://pool.viabtc.com/tools/txaccelerator/), but there are only 100 spots per hour. You need to submit your TX within the first second(s) to get it accelerated.
How does double-spending work ? How to do it with Electrum ?
If you had your fee marked as 'replacable' you can do Replace-by-Fee. Basically just setting a new (higher) fee for the
same transaction.
You can find out wheter you have RBF activated by right-clicking on the unconfirmed transaction. If you can click on 'Increase fee', its activated and you can easily
increase it by clicking on it.
This is the easier way of getting your transaction unstuck. But if you don't have RBF activated you can perform a double-spending.
Here is a guide for electrum:
https://steemit.com/bitcoin/@profitgenerator/tutorial-how-to-fix-unconfirmed-transactionsYou may activate RBF for your future transactions: Tools -> Preferences -> Fees -> Enable Replace-by-Fee. And tick 'Replacable' when sending a TX.