1. Use the drop down next to the font Family to select a new Family. You'll only be shown a list of fonts that are available to you:
2. Figma will attempt to match the Style to the missing font, but you can use the dropdown next to the Style to change it to another Style.
3. Complete the process for any remaining missing fonts.
4. Click Apply when ready to complete the process:
5. All text objects in the File will be updated to the new fonts.