How to change logo from backend?

From Magento 2.1 admin structure are changed,

All Design configuration are moved to Content -> Design -> Configuration section

You have to go for Admin:

Go To,

Content -> Design -> Configuration

Now go to your current theme,

Click on current theme you have to display new page opened, Inside This page, You have to go for Header section at below of page,

Inside Header you can add your Logo Image section.

I am using Magento 2.1.5 and have found header section from where we can change logo as below:

Admin => Content => Configuraton :: Select right record, you should see Header link in the left column.