Even if you update your game, there is simply no surefire way of making Leif appear on the same day of the update. It's best to wait for him to come visit your island. He will not come every day, so it's a matter of being patient.

Will Leif visit again?

Leif's Visit Schedule

Leif will visit your island randomly every two weeks!

Will Leif be permanent?

Ongoing events like the Fishing Tourney or Bug-Off will block Leif's space too. Unfortunately, this means that Leif does not currently have a permanent location on Animal Crossing: New Horizons islands, so you'll have to just hope that he shows up to shop at his store.