Iam underweight because of my poor digestion.how can i improve my digestive system?

Digestion: First, get a thorough physical exam by pcp or a gastro enterologist, to determine the cause & target that for treatment.

Complex Issues: Unclear what you mean by "poor digestion". Ingesting raw material from other life, breaking it down to small molecules we can absorb into the blood & lymph system combined with protecting our body from unhealthy things are complex, partially understood issues. A common basis for problems are internalized fears expressed into the intestines, eg "butterfly" sensations prior to test not prepared for.