Changing Rollover IRA Fund

I have a Fidelity Rollover IRA account that I have waited too long to figure out my best options. It currently is yielding basically nothing because it is in the FDRXX fund (cash reserves) - I want to switch to FZROX for the zero fee. BUT - I am unclear as to how I actually do that. Do I go in and buy that amount currently in the account? It just automatically buys it with the money currently in the account? Or do I have to sell the current amount and then buy? I just don’t want to mess up and end up being taxed on the amount when all I want to do is take the current amount and put it in a different fund. It wouldn’t let me exchange them. Thanks - I hope this makes sense…

If it’s in an IRA, buying and selling aren’t taxable events. Only if you withdraw the money.

Thank you! Should I be keeping this rollover IRA or converting it to a Roth IRA or something else? I also have a 403b from a previous job, but I stay home with my kids now, should I be primarily investing in that account instead of the rollover IRA?