How to blur some part of moving thing in a video editing?

Hi ​@Lokananda, 

Unfortunately, blurring or masking a moving object in a video isn't currently supported in Adobe Express; it's focused on simpler content creation and doesn't offer tracked masking for video.

For this, you'd want to use Adobe Premiere Pro, as it can do exactly what you want. If you have a Creative Cloud subscription, Premiere Pro is included in many plans and would be the recommended tool for this use case.

Hope that helps!

Best!