How to rename an attachment?

You can't rename attachments after their upload to the Jira instance. This is a known issue caused by the Jira bug - JRACLOUD-25060. 

How to create labels in the administer section after the app install?

Jira admins can create labels in the Label management section. Other users can create labels from Jira issues if they are in the label-creators group. You can find the detailed instruction in the Tag attachments with labels section. 

Can I access the REST API documentation for Smart Attachments for Jira Cloud?

We're unable to provide the public REST API documentation for Smart Attachments for the Jira Cloud due to Cloud platform's peculiarities.