Most begonia like being slightly root bound. Repot when new growth becomes less frequent, and roots stick out the bottom of the pot. Move up one pot size if possible. Use similar or identical soil to what it's growing in now for best results.
Spring or summer. Don’t do it on winter and autumn as the plant will suffer a bit more and be more shocked. Method wise I’m not sure, I transplanted mine and she’s having quite a bad shock 🫣
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