Add your multiple columns with comma separations:
UPDATE settings SET postsPerPage = $postsPerPage, style= $style WHERE id = '1'
However, you're not sanitizing your inputs?? This would mean any random hacker could destroy your database. See this question: What's the best method for sanitizing user input with PHP?
Also, is style a number or a string? I'm assuming a string, so it would need to be quoted.