On iOS devices:
Go to the menu ( via the three lines on the left in your expenses overview ) and click on settings:
The version shows ( in this example 5.9.0 )
on Android devices:
Go to your settings as well, and tap on settings
The version shows ( in this example 5.9.0 - 5454 )
In the relevant app store you'll always find the latest version, so you can compare your version with that one.
Rydoo strongly advises to use the latest app version, if you do not have auto updates enabled, make sure you make the update happen yourself or delete/reinstall the app so you have the latest version. You will not lose data unless your expenses were still uploading and you turned off the "save images" - option that is activated by default.