The Respawn Timer and Reset Feature, located on the top-left of your screen (screenshot below), Can only be found when you are in an area where there are mobs awaiting respawn and you are currently out of combat. You can Left-Click on this icon to open up the Respawn Timer and Reset Menu. Furthermore, you can Left-Click on “Respawn now” to immediately respawn all mobs that are listed in the Menu.

Tip: You can keybind the “Respawn now” feature to a hotkey in the Options for the game to save yourself a couple clicks.

— Sylynn