Go to Website > Website Settings.
For the “Powered by ERPNext”, you can edit here: frappe/erpnext/blob/develop/erpnext/templates/includes/footer/footer_powered.html
To disable “Your email address – Get Updates” subscription form: Website > Website Settings > Footer > Hide Footer Signup.
Google Tag Manager / Head Scripts
Important: Frappe/ERPNext has several parts. When talking about Website, it means the public-facing part, and that does not include Desk. The way to put scripts are different whether you just want to put them on Website only, Desk only, or both.
To put on Website only’s <head> HTML, you can go to Website > Website Settings > HTML Header, Robots, and Redirects > <head> HTML. Additionally, Google Tag Manager requires “after <body>” script, that you can put in the “Banner HTML” section.