Hi Ellen, are you trying to change a link to an existing page, like changing [[MyPage]] to [[MyOtherPage]]? If so, you can just rename MyPage in Obsidian and it will automatically update all your links to it.
If you’re trying to search-and-replace some other text across all the pages of your vault, I don’t think Obsidian gives an easy way to do that. You’d need to use another tool like sed, Notepad++, VSCode, etc.