Do not try to flatten it with weights or clamps. A distorted lid needs assessment of the structure and sealing arrangement.
What the observation tells you
First distinguish visible packaging beneath the lid from a change in the lid itself, using ordinary access only. Extra force can damage hinges and hide a persistent gap. A seal replacement alone may not correct structural distortion.
A useful next step
Keep contents suitably protected and record the shape with a safe photograph. Explain any impact, heavy object or handling event that preceded the change. Let a competent repairer determine whether suitable work is possible.
Manuals and supporting guidance
Model-specific instructions take priority. These references support the practical principles discussed here; they do not diagnose your appliance.