What advice should Lena give walter from a raisin in the sun to change the outcome of the novel? i need help with idead so I could write a letter to him?

I don't think that Lena would write him a letter. I think she would sit him down in a kitchen chair and have a "talk".

Check the description of Mama at the site below... there is an excellent paragraph on what Mama wants for Walter.
In stead of a letter... you might write a "dialogue" for a skit between those two characters.

http://www.sparknotes.com/lit/raisin/canalysis.html

the assignment wants me to write a letter

Then use that site to give you an idea of what Lena would say... Then think of how your Mom would write to you; that should give you the kind of letter Lena would write.

To provide advice to Lena from "A Raisin in the Sun" in order to potentially change the outcome of the novel, one approach is to consider the central conflicts and challenges faced by the characters. Lena Younger, also known as Mama, is a determined and resilient character who plays a pivotal role. Here are a few ideas that you can consider for your letter to Walter:

1. Encourage empathy and communication: Lena can advise Walter to develop a better understanding of the viewpoints and feelings of other family members, such as Beneatha, Ruth, and Travis. Stress the importance of open and honest communication as a means to resolve disagreements and bridge the generational gap.

2. Promote responsible decision-making: Walter struggles with his desire for a better life and financial security. Lena can advise him to make decisions based on practicality and responsibility rather than solely chasing dreams. Stress the significance of considering the long-term implications of his actions, such as investing in the house rather than in risky business ventures.

3. Emphasize family unity: In the play, conflicts arise due to differing aspirations and endeavors within the family. Lena should remind Walter about the importance of family unity and encourage him to prioritize his family's well-being. Suggest that he reflects on the impact his choices have on the entire family and the importance of interdependence.

4. Advocate for education and personal growth: Lena can remind Walter about the significance of personal growth and education. Encourage him to support Beneatha's aspirations to become a doctor and remind him of the positive impact it can have on their family's future.

5. Highlight the power of resilience and perseverance: Throughout the story, the Younger family faces numerous obstacles. Lena can advise Walter to persevere in the face of adversity, reminding him that setbacks do not define his worth or the potential for a better future. Encourage him to embody resilience and determine to make the best choices despite the challenges.

When writing the letter, it's essential to use specific examples from the play to strengthen your points and make them more convincing. Use your understanding of the characters and their situations to provide meaningful advice that Lena could genuinely offer Walter.