Use 301 Redirect In Google Blogger/Blogspot

Yes, Google Blogger does not offer you to edit .htaccess file but they just released an option to use Custom 301 Redirects. From this featurs, you can do what you want. Just follow the below step to use this features.

1. Go To Your Desired "Blog" Dashboard.
2. Go To "Setting".
3. Go To "Search prefences".
4. Click "Edit" On Custom Redirects ? no items Edit
5. Now Add Your Desired Old Post URL In "From" Input Section And Your URL Should Be Like /YEAR/MONTH/POST-TITLE.html
6. Now Add Your Desired New Post URL In "To" Input Section And Your URL Should Be Like /YEAR/MONTH/POST-TITLE.html
7. Check "Permanent" Option.
8. Click "Save".
9. Then Click "Save Changes".

I added a screenshoot below to understand completely.