Dungeon resets are an important part of Diablo 4, letting you farm them for XP, gold, and the chance of Legendary Gear dropping from elite enemies and bosses. That said, things have changed since the beta and there's no longer a simple button you can press to reset dungeons.
If you're just starting your journey in Sanctuary, you might want to know how to get your mount so you don't have to traipse around the map on foot. Otherwise, here are the methods I've found so far for resetting Diablo 4 dungeons.
Diablo 4 dungeon reset: How to do it
While the beta let you open up your quest menu in the map and use a handy "Reset dungeons" button to respawn enemies and bosses, sadly, this isn't in the game any longer. If you want to reset a dungeon now, you'll have to leave the dungeon. However, this doesn't seem to work if you portal to Kyovashad or another town—I guess that makes sense when your portal would lead you back to the boss arena or a stage of the dungeon that would be locked after a reset.
Instead, you've got to head back to the dungeon entrance and wait for a few minutes, which resets all of the enemies and the boss. You can either run back there yourself or simply use the leave dungeon option in the emote wheel. You might also be able to reset via portal as long as you portal out of the dungeon in its first section, i.e an area that doesn't require dungeon progression to access, but if you can, it takes a very long time to reset.
Your best bet overall for farming dungeons is to pick a short one with a high density of elite enemies, like Anica's Claim in the Fractured Peaks. That way you can blaze through to the boss, collecting XP, gold, and loot, and then quickly leave to do it all over again. You also won't need to portal back to any town often to sell or dismantle because there's a blacksmith right outside, though you will need to complete the Malnok Stronghold to get access.