If you publish your workbooks to a Tableau Server, then you need to make sure the Tableau Server environment in your organization is already on 10.5.
You won't be able to Publish to a lower version of Tableau server from Desktop V10.5 since 10.5 involves .hyper extracts which won't be supported by older Tableau server versions.
If you do not publish and have standalone Tableau Desktop, you can install 10.5 and make a copy of your workbooks and open them in 10.5 and test out all the features.
Since saving any workbook in 10.5 converts that workbook to version 10.5 you won't be able to revert back to and older version. so having a backup copy of the workbooks(created/saved in 10.3) can be useful if you want to revert back to 10.3
Hope this helps.