Go to the SharePoint Site Contents page:

Select the page and add your report:

Click the gear icon on the right, and then Edit page:

Under Categories<- Media and Content. Parts<- Content Editor, and Add:

Select new content. From the menu, Format Text<- Edit Source:

In the Edit Source window, paste your iFrame code in HTML Source, and then select OK:

Page, <-Stop Editing:

For more information on Power Bi, check the following Power Bi Training