Migration tool which comes with tally 9 has to be used for this purpose. It is also available on the right hand side bas under download section. Few things have to be taken care here also:-

1. Migration tool works with original license version. First it check the license file and then proceeds.
2. You can migrate from backup taken in Tally 7.2.
3. In case of unsuccessful attempt or error message in migration process you can check migration.err file which is created in same folder after migration process ends migration.log files can also be seen for details of migration process.
4. Win 98 can cause you some problem while migrating so XP or XP home is better