Bought a send hand system that you're locked out of? Don't want to deal with nintendo and get right into using your new console? I can't blame you so wait no longer with these easy steps!
This will also work with Wiiu, and Switch, I own neither of these so I can't give you a step by step guide on how to do this, but I can't imagine it's much different.
The steps for 2DS are exactly the same as the 3DS. And should be similar for DSi. My DSi Broke so I can't test this here either.
Step 1: Click on the wrench icon (Which is the settings app.)
Step 2: Click on parental controls (The icon that has the parent and child logo, self explanatory.)
Step 3: Click 'forgot pin'
Step 4: If you know the answer to the security question, then simply answer it and you're set. If you don't then continue to step 5.
Step 5: Once you click 'I forgot' in response to the security question you'll be given an inquiry code.
Step 6: Click here
Step 7: Enter the current date your DS is set to. Year does not matter and does not need to be altered. THIS IS NOT THE CURRENT DATE UNLESS IT MATCHES THE DATE ON YOUR DS.
Step 8: On your DS system hit "ok"
Step 9: Enter the inquiry code into the website above
Step 10: You will receive a master code
Step 11: Enter the code into the DS and accept
Step 12: You may now change the pin!
Q: It's not working! What do I do?
A: Set the date to 1/1. Year doesn't matter, and I don't recommend altering it. Try steps again after changing the date.
Q: It still doesn't work!
A: It might be down to your firmware update. Though this hasn't presented an issue and my 3DS is up to date at the moment of creating this guide
Q: Will we ever get a specific guide for the other systems you mentioned?
A: If I ever own them, yes.