You sure can!
Go to your uploads page and identify the file you want to delete. Click on the file and you'll get an info popup. At the bottom there's a button to delete this file. After a confirmation step, this file — and all associated contest entry data that came from that file — will be deleted.
What you can't do from within RotoTracker is selectively delete contest entries, such as deleting all data prior to a specific date. We recommend you simply upload all your data, and use RotoTracker's powerful filtering to only look at the data you're interested in. If you really do need to delete a specific subset of your data, you will need to delete the file as above, than manually make the changes to your CSV file before uploading.
If you can't find the answer to your question, or you have found a bug, have a suggestion or have feedback, please don't hesitate to contact us! You can email us directly at email@example.com and we'll get back to you!