How do I upload a new theme in Blogger?

First you'll need the new theme, which is called a "template" in Blogger is XML format saved in your computer.
Then follow the steps below,

1. Backup your old template. (In case something is wrong with the new template, you can revert to the old template)
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2.GO to https://www.blogger.com
3.  login your account
4. Click on blog you in which you want to upload new theme
5.Click on "Template" button that is on the left side.
6. Click on "Backup/Restore" on top right corner.
7. Click on download full template ( This will backup your old template)
8. Now click on Choose file and select the XML file you want to upload
9.Click Upload and you're done. Check if the new theme works properly.