I want to change my B1 Visa application to B1/B2 Visa before the interview. I don't know if it's possible.
You won't need to.
A large majority (about 98 percent) of visitor visas to the USA are issued as combined B-1/B-2 no matter whether the applicant specified business or tourism in the application.
If you end up getting a B-1 visa only, it will be because there are particular circumstances to your application that give them cause to deliberately select the restricted kind of visa. In that case an application for a combined B-1/B-2 visa probably wouldn't have been successful anyway.
Note that if you have a single trip that combines business and tourism activities, then this is allowed even on a pure B-1 visa. There is no combined B-1/B-2 immigration status anyway, so business travelers arriving on a B-1/B-2 visa will be admitted in B-1 status. And even the immigration police does not require that business visitors refrain from enjoying themselves during their trip.
You do not need to apply to change your nonimmigrant status if you were admitted into the U.S. for business reasons (B-1 visa category) and you wish to remain for pleasure before your authorized stay expires.
The only situation in which you need a B-1/B-2 visa specifically is if you want to use the same visa for different trips, of which some are business and other are purely tourism.