How to change Piano to Church Organ as an option to Play back my Song

MuseScore scores don't support "options". You play them as they are, or you change them so that they are different. "Options" might be a nice idea, though ...

There is a "Church organ"/"Pipe organ" among the keyboard instruments. As an organist, I can tell you that playing something meaningfully and stylistically on a real (pipe) organ is not simply a matter of playing the same notes and same keys as you might on a piano, and the matter of emulation of pipe organs on computers, in particular MuseScore, is quite complex, and presumes knowledge of the structure, function, and use of pipe organs. But, for what you're doing, the "old MuseScore Pipe Organ" just switched in as a "patch" (sound type) might be more than enough. Try it and see.

Either choose that voice in the Mixer (F10) or use a "Change Instrument" text (from the Text) palette, change properties to "Pipe Organ".

To be clear, though: you can use any instrument you want to play back a score, you certainly aren't limited to piano just because the default empty score happens to use it. When creating a new score with File / New, you'll see dozens of templates and hundreds of instruments to select from. Or, if you've already created a score for piano, you can change the instrument by right-clicking the staff, Staff Properties, Change Instruments, and you'll have the same hundreds of instruments to choose from.

I think the problem is a bit deeper than organ vs. piano.

At some point pretty soon you're going to need to make a decision about whether peaceful, gainful employment is more or less important to you than the ideals of Roman Catholic Church Music.