Time traveling is actually really simple and has minimal consequences in-game. To change the time in-game, all you have to do is change the time on your Switch.

Go to the home menu of your Nintendo Switch and select the settings button, then navigate to "date and time." Disable "Synchronize Clock via Internet" and then manually enter the date and time you want to travel to.

Once you've done that, just reopen the game and you'll be in that new time! It's important to note, though, that if you plan to buy or sell turnips then you will not be able to time travel, otherwise they will rot and be unsellable. You also will be unable to travel forward to future big events, so don't use this to access a major holiday early or you'll be unsuccessful.