Significant other needs to get treatment. CPAP, for instance. Lose weight, if applicable. Surgery if necessary. Address the root of the problem because it can be a significant health issue for them, while also impacting your health via mechanisms related to lack of restorative sleep.

If it’s just a once in a blue moon thing, and waking them up / having them change positions doesn’t resolve it, I will go sleep in another room, put in ear plugs or listen to some relaxing music with earbuds in to drown out the snoring.