It's determined by the song you are on. If you started a song with 44.1 khz sample rate then when that song project is open, it will set your hardware back to 44.1. If you want to set and use a different sample rate, you will need to do that for a new song project. As far as I know, there is no easy way to convert an entire song to a different sample rate, but maybe there is. I just don't know.
EDIT: I stand corrected. You can change the sample rate of a song. Go up to the Song menu at the top (it's between Edit and Track in the top menu bar), go to Song Setup, and in the General tab of the General menu, change the Sample Rate to your desired setting, then click Apply at the bottom. Not sure if this has any implications on the sound quality and/or performance of the tracks within the song, but yeah, it can be done.
I know if you drag and drop a track that is in one sample rate into a project that is set into another, it will convert that track to the new sample rate, so perhaps you can start up a new Song and drag and drop the tracks in, one at a time?
