To switch to Sandbox Mode, click on the hamburger (3 horizontal lines) at the top of the left menu, next to the Payment Rails icon. This will expand the left main menu.
At the bottom of the left main menu, you will now see a toggle switch indicating if you are currently in LIVE or SANDBOX mode. Simply, switch to the one you want.
Sandbox mode is also indicated with a yellow sandbox marker at the top of each page:
Please note that some settings are common across Live and Sandbox mode, in order to provide a "production-like" testing environment.