I’m not sure what theme you are using but typically, if you get a notice that a theme you have been using has been updated you should update it as it may include fixes, or new items, security upgrades, etc.
That being said, if you have made ANY custom changes to the theme (on any pages, or images), updating it will of course overwrite them.
Updating downloads the new theme you and completely copies the files over the current theme. If your theme changes are made on the template and not through an administration backend through the WordPress backend it will be over-written.
You should backup your current theme before updating and save it to your hard drive, and in the future if you are going to make changes you should consider making a child theme out of it, or adding in a secondary stylesheet that will not be overwritten for future upgrades.
