Click the box to place the cursor inside it
Go to the Developer tab
Click Check Box Content Control
If the box is a symbol, click it and type an X or check mark if editable
Insert a check mark symbol from Insert > Symbol if needed
Use Alt + 0252 for a check mark in some fonts
Use Alt + 9745 for a checked box symbol on Windows
On Mac, use Option + V for a check mark
If it is a form field, click or double-click the box to toggle it