Have you tried to simply push on it? Mine did that. It's just held with clips. Went back on in the time it takes to hit your vehicle.
I actually pulled on it first, removing it ever so slightly more, to observe where the clips were to make sure they were not broken and were lined up so I did not break them.
Bang it upwards with your open palm. It should snap back into place. This is French from @antirouillebolduc in Quebec who provides rust protection and 3 sizes of Model 3 mud guards but will show you exactly how the lower plastic rocker cladding is attached: