Water only when the soil is 100% dry. You want to err more on the dry side than too wet. Be sure to check the soil before watering. Water thoroughly until you see it flow out of the drainage hole. Discard any excess water in the saucer. This ensures your plant isn’t sitting in water which can lead to root rot.
Your euphorbia needs bright light. This means at least 6 plus hours of direct sun per day. Too little light and your euphorbia is not able to properly metabolize the water in the soil leading to root rot.
Weakened or stressed plants become more susceptible to insect infestations. Sap-sucking bugs like spider mites, mealybugs, and scale can drain your plant of moisture. This problem quickly manifests itself by yellowing leaves. If not killed early on, these small pests proliferate and move all along the plant into nooks and crannies. The piercing mouths of the insects exhaust your plant and accelerate yellowing, especially if your plant is already unhealthy. If you discover any pests, treat with neem oil or insecticidal soap as needed.