Just stand quietly, presumably. That's what I do, as a republican. I also decline to sing 'Onward Christian Soldiers', as a Christian(ish) pacifist(ish).
Indeed - but how stupid is it to have a national anthem that probable one third of the population disbelieve or disagree with on the basis of the five five words alone. Probable more as the roughly one-third of the population who don't believe in god and the roughly one-third who want to get rid of the monarchy aren't entirely overlapping!
Dumb in the extreme.
