It somewhat depends. Is this an actual synonym of the taxon? Or was it a true error? The synonym name appears to have been added to it automatically based on Catalogue of Life. https://www.inaturalist.org/taxon_names/1395664/edit
If it was just an error, you would delete the synonym name on the taxonomy tab of the taxon page and create a new taxon for it.
If it is a taxonomic issue, you’d create a new taxon for it, a new taxon for Erastia salmonicolor sensu stricto, and create a taxon split for E. salmonicolor sensu lato split into E. salmonicolor + Sarcoporia polyspora.