I'm not sure what you're asking (Is English a second language for you?), but:

* If you don't want to change which companion you are using, you don't have to, unless you are doing a story segment that requires a specific companion.

* If you *do* want to change which companion you are using, press "N" (or its equivalent on a keyboard that isn't US/UK QWERTY / French AZERTY) to open the "Companions and Contacts" panel, and press the little arrow to the right of the companion you want to use. Your current companion will disappear and the one you want will appear. It takes about three or four seconds.