I am wanting update the look on my website, but currently, for some keywords my website ranks very well in Google. If I update the website, how much will it effect the rankings on Google?
I don't think changing site's look will have effect on PR of your site. Because the PR is effected by your site's content or SEO, not look of site.
If you keep the old content, changing only the design, but not the internal links than your ranking will be the same.
When my boss changes the website components for stronger keywords and had removed the www on the homepage I lost my Google rankings.