I want to modify SharePoint online new experience new entry form. Is it possible? If yes then how?
You can not
Microsoft disabled all branding/customization
Raise your voice here:
Recently, we've released a tool for designing fully-responsive SharePoint forms and publish them to modern pages - Plumsail Forms.
Our forms are totally customizable - you can add your own JS- or CSS-code into forms and our rich JS-framework will help you with that.
But even if you do not have skills in programming, you will still be able to design cute forms with our handy designer as simple as dragging and dropping fields and distributing them in grids, tabs, and accordions.