Need Guidance For Upgrading Mautic With Custom Configuration In The System

I am currently Using Mautic version 3.2. I have made many modifications to default system according to requirements. I want to upgrade my Mautic system from version 3.2 to 4.4

Please note, There are many configurations and code level changes which I want to keep in the newer version after upgrading the system.

Kindly suggest me or guide me with proper steps and procedure to upgrade Mautic system.

Thank you