As Achiel mentioned, the functionality to roll back/revert to a previous revision does not yet exist in the Modeler. However, you can use TortoiseSVN to achieve what you want.
Use TortoiseSVN > Show log to find the revision that you would like to revert to. Right-click that revision and choose 'Revert to this revision'. TortoiseSVN will apply all changes needed to revert to that revision in the working copy. Then, you can open the project in the Modeler and, if you are happy with the result, commit the changes.
Please note the following:
In addition to Benny's answer:
Note that you might need to revert the changes of multiple revisions. Let us say you are at revision 40 currently and you want to go back to the way things were at revision 37. You cannot delete revisions from the repository, you can only make new ones. In this case you want to reverse the changes of revisions 40, 39 and 38. The model will then look the same as it did in revision 37 and you can commit this as revision 41.
I'm assuming you're referring to version control in the modeler here..?
If that's the case, you can't do this from the modeler yet. You can work around this by using TortoiseSVN