Always keep backup of your theme files and database before theme update
Go through ‘Update: Important Considerations’

Manual Theme update can be done :

• via FTP

Updating Theme via FTP

1. Get updated theme.
2. Extract the latest version of theme.zip file.
3. Go to “wp-content > themes” location
4. Keep backup of your current theme folder.
5. You can delete the theme files then. This will not delete data / content.
6. Drag and drop the new theme folder into “wp-content > themes” location.
7. Done!