Redshift Insert into s3 file data into existing table

Easiest way is to use COPY command. Details and examples are [here] (https://docs.aws.amazon.com/redshift/latest/dg/r_COPY.html).

Alternately, you create an external table using Redshift Spectrum and then use 'Insert Into Select from' in Redshift to load this external table data into Redshift core table.