You have rewritten the first two sentences correctly in the active voice. However, the last three sentences still contain passive voice. Here’s a fully revised version of your paragraph with all sentences in active voice:
Alex Muros won the science fair this year. His project explained why leaves change colors in the fall. The principal gave Alex a prize. He will also present his project at the statewide science fair in the spring. His parents will take him out for ice cream to celebrate his hard work.
Great effort! Just a few adjustments were needed to keep everything in the active voice.