I don't think you can right now... there was a thread not too long ago and SNM said that this version of the WM and 1RM you can not delete a test, however, you could always either "zero" out your results or change them... Why would you need to delete one? inquiring minds want to know
In the soon to be released beta 2.0 version of the WM you will be able to do this. You can't delete a test result in our current version, but you can retest.