If your website was designed more than two or three years ago then the chances are it is probably starting to look a little dated. As with anything technology based, advances are made year upon year and what was trendy and successful one year, may not be anymore.
If you work with a web design company or used a freelancer to design your website then you may decide to go back to them for updates. Sometimes the person or company you previously used no longer offer web design services, in which case you will need to find a new company to work with. IF this is the case, then it is vital that you have all the access to your current site. You need to ensure that you have the login details for the domain name and ideally the hosting too. If you cannot get this then you will need to start again with a different domain name and this can be a nightmare as you may have signage, letterheads and advertising printed using that domain.
Often just by changing the fonts, updating the images and effects and changing to colour ways, can make a huge difference to the appearance of the site.