To improve the domain authority of a website you have to do both on page and off page SEO. Build quality backlinks and do proper on page optimization. Good traffic will also bring traffic to your site.
Domain authority is the measure of strength and popularity of your domain in search engines. It is measured by Moz and your domain is ranked between 0 to100 depending on the popularity and trust. DA can be improved by good content update and also by building backlinks from other relevant domains.