To upload the template file in miro you need to follow the instructions from the Miro help page. Here an excerpt:

"To restore a board from backup, go to the Dashboard and click Upload from backup. A dialogue box will appear, where you will be able to choose your *.rtb board backup file. After you confirm your choice, the board will be restored."

If you use a free miro account

This feature is available only for the paid Miro accounts. 

But there is a workaround if you have a free account. I'll need to copy the template just for you manually. 

So if you have a free Miro account, just drop me a line in the chat bubble in the bottom right of the screen, and I'll reach back. This process will take a few days to do. 

Sorry for the inconvenience that comes from the Miro free tier limitation