There is a filter for sort options that you can use, but it looks like there is already a Sort By: Popularity option there. You can see the filter here.
You may want to check to see if other plugins or your theme are defining where the Sort By text is coming from.
WooCommerce also uses the built-in WordPress translation system, so you may be able to use something like Loco Translate (https://wordpress.org/plugins/loco-translate/) to find the string you want to modify and create a “custom translation”, from “Sort By” to “Sort By Popularity”.
Hope this helps!