The short answer is: You can't.
In order to ensure a linear workflow, you are not able to undo commits. If you want a data set to be deleted, you will need to delete the data set in openLCA and commit this deletion to the repository. The data set will stay on the server in the commit history though, but that is intended.
In practice, of course sometimes you need to undo a commit (e.g. you published data that is copyright protected etc). The workflow is:
a) If it is the first commit, simply delete the repository and create a new (empty) repository
b) If you want to go back one (or several commits), clone your repository and in the 'clone dialog' select the last commit that you want to keep, all commits afterwards will not be copied into the cloned repository. Afterwards delete the old repository and move the cloned repository to the old path. (e.g. repository: 'example/repo' => clone to 'example/repo_clone' => delete 'example/repo' and move 'example/repo_clone' to 'example/repo').
Note: If any user has already fetched your commit, they will run into problems when you do this. So you need to ensure that no one already fetched the data, or inform all users that they will need to: 'Disconnect' the repository in openLCA -> 'Connect' again to the repository and perform a fetch operation. This way they will be back in sync with the new (cloned) repository.