Uploading html on UI 7.1

Update on how I resolved this issue, Thanks to Ziggy for getting me this far:

UI 7.1 does not support Developer Mode and, at the moment, and cannot be added.
Because I created my account recently I was able to create a new site using 7.0, migrate over, and cancel my original subscription.  Of note you will need to purchase a new subscription PRIOR to cancelling your old one for a refund and the refund itself will take time to process.  To do this,

1. From the Site Dashboard go to 'Create Website'
2. Skip till you get to the template selection screen
3. Scroll to the bottom of the page and click "Looking for templates from an earlier version?"
4. Click a template and proceed through any required setup
5. In the left panel menu for the new site click "Help" and verify it uses version 7.0 at the bottom
6. Navigate to Settings>Billing>Subscriptions and create a new subscription that is a duplicate of your original
7. If you have any registered domains for your site they need to each be migrated individually from the old site:

7a. On the original site navigate to settings>domains>[specific domain name]>move domain to another site
7b. Select the NEW site you want to move the domain to
7c. Verify it migrates correctly and return to the old site to repeat 7a-7c for each domain you want to migrate

9. If you have any files or settings to export, this would be a good time to get them.  I did not so could not provide input on how easily this can be performed After you cancel your subscription.
8. On the old site navigate to Settings>Billing>Subscriptions and cancel the current subscription.  If it is within a certain amount of time from its creation the cost will be refunded to you at some time in the next couple weeks.