How to void a posted contribution
While you cannot void a contribution, you may either reverse it or delete it depending on the entry method you used.
If your contribution is in a closed batch, you will want to follow the instructions outlined in the Help Article, How to Edit Contributions in a Closed Batch.
If your contribution is not in a batch, you would first bring up the Contribution List, (Offering, Contribution List, date range).
Find the Contribution. To do so, you may use the filter fields at the top. For example, if you know the Envelope Number of the contribution you need to void, enter that envelope number in the Env # field and press the enter key on your keyboard.
Once you have found the specific contribution you want to void, click the Delete option which looks like a garbage can.