Find is the same idea as in every editor; the detail worth knowing is that it understands the script's structure, so renaming a character changes the cue and the lines that mention her without touching a word that happens to share letters.
Open find
Press CtrlF. A bar appears above the page with a search box and a match count.
The find bar above the page: the search box (1), the match controls with Previous and Next (2), and the Replace toggle (3) that opens the second field. Type what you are looking for
Matches highlight as you type. Press Enter to move to the next match, and Shift+Enter to go back. Toggle Match case and Whole word from the bar when a name is also an ordinary word.
Switch to replace
Press CtrlAltF to open the replace field, or select the Replace toggle at the left of the bar. (On Windows and Linux the same bar also answers Ctrl+H.)
Replace one, or all
Replace changes the current match and moves on; Replace all changes every match at once. Replacing all is a single undo, so Ctrl+Z takes the whole thing back.
Close the bar
Press Esc. The highlights clear and the cursor stays where you left it.