From 0.762 to 0.764 there are no DB changes, so just replace all files EXCEPT third party modules, custom blocks, custom themes AND config.php

Process:

1. Backup files.
2. Set theme to extralite.
3. Delete all files.
4. Upload version 0.764
5. Place back third party modules, custom blocks, custom themes.
6. Place back your old config.php

More info on PostNuke