Factory reset a 2013 iMac
How do you factory reset an i Mac from 2013
What are your plans for the 2013 iMac?
If you are selling, giving away, trading in or recycling,
read thru > What to do before you sell, give away, or trade in your Mac - Apple Support
If you are keeping it and just want erase and reinstall, follow the links in section 8 of the Support Article.