Enter the text you want to replace in the Replace: box. If you don't want to change text text but just affect its font, colour or size, remove the tick from the Replace: checkbox.
Un-check Replace: to leave the text as is, changing only it's font, size or colour and alignment. In this case, the Replace and Replace All buttons will be disabled until you choose a Font, Size or Colour change to make.
To set the Font, Size or Colour you wish to use for the replacement text, click on the reflow line, align auto hyperlink.
To do font replacement with this dialog (rather than using the Replace Fonts dialog), un-check both the Find: and Replace: check-boxes and set-up character formatting for both the find and replacement.