For beginners, I wanted to add to the accepted answer, because a couple of subtleties were unclear to me:
To find and modify text (not completely replace),
In the "Find" step, you can use regex with "capturing groups," e.g. your search could be la la la (group1) blah blah (group2), using parentheses.
And then in the "Replace" step, you can refer to the capturing groups via $1, $2 etc.
So, for example, in this case we could find the relevant text with just <h1>.+?<\/h1> (no parentheses), but putting in the parentheses <h1>(.+?)<\/h1> allows us to refer to the sub-match in between them as $1 in the replace step. Cool!
Notes
To turn on Regex in the Find Widget, click the .* icon, or press Cmd/Ctrl Alt R
$0 refers to the whole match
Finally, the original question states that the replace should happen "within a document," so you can use the "Find Widget" (Cmd or Ctrl + F), which is local to the open document, instead of "Search", which opens a bigger UI and looks across all files in the project.