After setting a password, attempts to visit UpdraftPlus’s page will be met with a password prompt:
Here are the add-on settings; you can choose how long before a user has to re-supply the password (from 1 hour to 1 year); and you can customise the support link given on the password screen.
Note that this add-on is not intended to be a comprehensive solution for your security. Ultimately, someone with admin-level access to a WordPress dashboard has the right to do anything – for example, they could install their own plugin to do anything else. This add-on is not intended to lock out WordPress experts, but to be a useful way of protecting your UpdraftPlus settings and backups against meddling and casual nosiness.