If you don't want (or are unable) to walk those 10-15 min a bit uphill, from the main bus stop on Via Bilenchi, where the 131 from Florence drops you off, you can catch the local urban bus no. 202 which can take you right in the city center. I think you would need to buy an urban ticket for that (1.70€ for 70 min validity). Details on where to buy the urban tickets are in the AT website. It can be tap and go, or SMS text, or via App, or if you prefer also paper ticket purchased at the vendor (like the ticket office at the Florence bus station).
Edited: I had mistyped. It’s bus 202.
https://www.at-bus.it/en/travel/lines-and-timetables/siena
(Select COLLE VAL D’ELSA URBANO and then 202)
For the 131 from Florence select Altri Servizi/REGIONALI, then 131
https://www.at-bus.it/en/travel/lines-and-timetables/firenze