Sometimes you will see the "use classic mode" header that shows up at the top of the page, but the easiest way is to go to the CRM URL directly (yoursite.crm.dynamics.com) and then click on the gear icon in the upper right corner and then "Advanced Settings"
Then click on the arrow next to "Settings" and then either click on "Customizations" or "Solutions"
In my case I clicked on "Solutions" and then clicked on the solution that holds that form I want to edit, then dig into the entities, then the entity (account in this case) and then "Forms"
Then click on the form you want to edit.
If you click on "Customizations" then you will need to click on "customize the system" and you will be editing the default solution for the environment.
