Please follow some step
Singup with valid id and password
Google update blogger dashboard theme that's new ..don't worry click on setting link it's displaying right side.
Select old blogger dashboard then it's displaying old version for blogger admin section (own blogger admin section)
Click on design
Click Edit HTML
Before upload new theme please download old theme ...click to download full template.
After download , click browse option then click upload
After upload it's display keep widget option, delete option and cancel option
Click according to you and before save check preview, if its display fine site running that's fine save theme.