This is not easily possible unless you've created a branch before adding said widget.
If you haven't made a branch beforehand (which i'm guessing is the case):
On the forum there's more questions about this. And there are some posts where some people are detailing how they have succesfully downgraded in revisions by using Tortoise SVN (version 1.7.15 for use with Mendix).
But i have no first hand experience with this.
You can start a new branch line based on an older revision of the main line. See the screenshots on how to achieve this. In this way you can continue working from an older revision of the main line.
You can create a branch based on an old revision and work on that, as described in other answers. However, merging that branch back will not make the main line be like that old revision because only the changes made in the branch will be merged.
The proper way to go back in time, so to speak, is to revert the commits you made after the commit you are interested in. You can do that through the Modeler by using Project > More versioning > Reverse merge changes here. Select the revisions you want to forget and all those changes will be done in reverse bringing you back to the old state. Then you can commit and continue working.