How to reset restriction code without computer?

See the following from Use parental controls on your iPhone, iPad, and iPod touch - Apple Support

If you forget your Restrictions passcode, you need to erase your device, then set it up as a new device to remove the Restrictions passcode. Restoring your device using a backup won't remove the Restrictions passcode.

If you click on the link in the above it gives instructions on how to erase the device and the iCloud method does not require a computer. The other option if you don't have a computer is to go to an Apple Store or service provider to perform the "Erase your device with recovery mode" option.