In a 6-qt. saucepan, heat milk over medium and attach a candy thermometer to the pan. Cook the milk until it reaches 175°. Reduce the heat to low and maintain the temperature for 10 minutes. Remove from the heat and let stand until the milk cools to 98.6°, about 45 minutes.
Whisk in store-bought skyr and rennet; then cover and place the pan in a cold oven. Let the milk stand, undisturbed, for 3 hours. Uncover and, using a knife, cut the curd into 6 to 8 large pieces. Cover the pan and return it to the oven; let stand for 3 hours more. Transfer the pan to the refrigerator to chill at least 12 hours.
Using a ladle, scoop the curds into a cheesecloth-lined strainer set over a bowl and cover with plastic wrap; discard the whey. Let the curds drain in the refrigerator for 2 to 3 days (the longer it sits, the thicker the finished skyr will be). When ready to serve, scrape the drained curds into a bowl, and stir in herbs; or sweeten with sugar and serve with fruit.