I have plotted the exoplanet curve in siril, but now how can I fit the model so that it may be better visualized.
You can’t just now, because Siril doesn’t got the function yet (and I don’t think they are planning to do so for the time being…).
You could learn a more dedicated software like astroimageJ (some times I use Siril for preprocessing/stacking and then I pass the sequence to AIJ to measure, make tables, etc.).
Better way is using EXOTIC. For that, you could join Exoplanet Watch community in slack where you will get a lot of help from us.