To access an existing requirement for editing/deleting, go to...
Analysis -> Check Requirements...
...where you can select any of the current Requirements and either edit them, break there link to an expressions, or delete the Requirement altogether.
Note that the reason this is done this way and not from inside the expression system is two fold. For one, contrary to what it might look like, an Expression doesn't actually OWN the requirement, but rather the other way around. Granted, you can create a Requirement from the Expression system but in reality what is happening is that we are simply using the Expression system as a way of getting to the Requirement CREATION/ASSIGNMENT mechanism, but not access to the editing or deletion options. Note that we did this to streamline the process of creating a new Requirement rather than forcing a user to go to a separate 'Define a Requirement' sort of function before he can assign it to an Expression (which he can do if he wished by going to the 'Check Requirements' dialog first), but what's really happening is that a Requirement is being created and THEN the assignment of an Expression is being made, IN THAT ORDER.
What you've already noticed of course is that once created, a user can NOT edit or delete a Requirement from within the Expression system and that is BY DESIGN. What this does is allow a company to remove the 'Check Requirements' menu item from the Analysis pull down menu which would create an environment where existing Requirements in a model can NOT be altered or removed by someone who perhaps is not 'authorized' to define Requirements but who is still expected to abide by and honor the Requirements which are already in a part design.
Anyway, I hope that helps as well as make it a little clearer as to why we arranged the different aspects of managing Requirements in the way in which we did.
John R. Baker, P.E.
Product 'Evangelist'
Product Design Solutions
Siemens PLM Software Inc.
Cypress, CA
To an Engineer, the glass is twice as big as it needs to be.