If you just want to migrate the databases you can use [Express Migration Tool 3.1][1], which can help you to migrate the data from version 8.1 to 9.0 Initial Release. While running the tool just select the option below:

[![enter image description here][2]][2]

Since the express migration tool does not support any version above 9.0 Initial release as a targeted version. Hence after express migration you need to run the SQL scripts mentioned in the upgrade guide from 9.0 to 9.1 Update 1. 


  [1]: https://dev.sitecore.net/Downloads/Express_Migration_Tool/31/Express_Migration_Tool_31.aspx
  [2]: https://i.sstatic.net/TjqIl.jpg