给定一个段落和一个单词，将段落中所有的该单词替换为指定的另一个单词，并返回替换后的段落。

原始段落为：“I love going to the beach on weekends. It's my favorite place to relax." 替换所有的"beach"为"mountains"
替换后的段落为：“I love going to the mountains on weekends. It's my favorite place to relax."