Safely modify plugin

Hello,

What is the best way to modify your plugin? Without loosing changes after upgrade or update.

For example: I would like to add additional custom button inside “entry detail” page next to: “Move to trash” – “Print” buttons. With additional functionality.

Any guidance or advices are welcome.

Sincerely,
Mintaras

https://ww.wp.xz.cn/plugins/visual-form-builder/