If you find that the exposure time field is uneditable there are three possible reasons:
The exposure is WFC3/IR MULTIACCUM and default is the only valid selection.
The exposure has at least one subexposure with an "actual duration" (override of default subexposure time). These could have been set by you, but most likely were set by using the autoadjust feature of the Orbit Planner. You can see (and clear) actual durations by selecting individual subexposures from the subexposure folder within the exposure folder. Alternatively you can reset all the actual durations in the visit using the "Clear Actual Durations" button in the Orbit Planner.
The visit has executed and cannot be revised.