There are a few reasons why your website should be upgraded or replaced. They fall into these categories:
Over time the purpose / emphases / objectives of a website evolve. Perhaps the primary rationale changes, or its target demographic changes. For example, instead of targeting all ages the site owner wants to target only young people. This shift could well require changes to the website’s language style, media and colour scheme.
A responsive website (one which displays well on all screen sizes) is de rigeur in 2017, but if your website is some years old it may not adhere to that principal. See this site for a webThemes creation which showcases responsive design. Or perhaps your site loads slowly or there are other technology or design problems.
Take a look at websites belonging to some of your competition. Try to be objective…how do they rank compared to yours in appearance, information offered and media used ( images, video, etc)? Are you using only stock photos or do you use images specifically taken of your business? Specific photos are more personal and relevant, showing your company culture.
Think about how you can differentiate between your website and your competition’s. A great site to view is awwwards.com which upends traditional website designs.
This aspect was covered to a degree in the previous point. It includes the issue of not overloading the prospect with information as people browsing the internet have short attention spans. Your site should be easy to navigate and each page needs a “call to action” such as a link to your contact page to make it easy to get in touch with you.
Consider your web site’s colour scheme. Ideally one primary colour together with one or two associated secondary colours should be used. See this webThemes article for more on colours.