First, I thought you wanted to play only on defense, which confused me. lol.
Just pause the game in practice mode and select "flip sides". Afterwards, you usually have to run one more play on offense and then it switches. That is why I select flip sides and then re spot ball which takes care of that.
Ahhh. I kept trying flipside, but it kept me on offense and I didn’t realize I had to run another play first