How to recover old iPhone passcode to open iPhone

Unless you wrote it down or kept it somewhere, there is no way to recover an iPhone passcode.

There is an Apple article that explains what to do if you have forgotten an iPhone or iPad passcode. Click on the following link: How to remove your iPhone, iPad, or iPod touch passcode without a computer - Apple Support

If that option is not available, you will need to use a computer. See: If you forgot your iPhone passcode - Apple Support