Where to update new Mosaico templates
When you install the Mosaico and related Extensions, you then create the Templates in civicrm and they become available to use in CiviMail, you do not upload them.

ADDENDUM:

If you have installed correctly, as per instructions, ie Shoreditch then FlexMailer then Mosaico then at /civicrm/a/#/mosaico-template you should see the word NEW on top of the base template