If you are using Virtue Premium, Ascend Premium or Pinnacle Premium you can find built-in options to change your search placeholder text within the theme options. Navigate to Theme Options > Language Settings, and change to whatever you need:
View your search bar to see the change:
For other themes or searches created from a plugin check out this tutorial for text changes of any kind: https://www.kadencewp.com/knowledge-base/recommended-string-editor-say-what/