Really? You are so off .
You would need some scripting knowledge as any of the default bitcoin core/qt or other wallets won't allow you to easily execute a double spend.

Any particular reason you want to try this? Just to see how resilient the protocol is? Short of having a ton of mining resources at your disposal, it's going to a bit hard to pull off even a 0 confirmation double spend since most miners will reject the transaction by comparing what's in mempool. For a 1-3 confirmation doubles spend, the difficulty becomes exponentially higher.

In short, there's no GUI 'test a double spend' client that I'm aware of.