A form cannot be deleted as such, but must be archived. This ensures that the data collected through the form is retained, even if the form is no longer in use.
You need to make sure that the form is no longer in use. If you are unsure, export a dataset using the form’s ID and check when new data was last entered into it.
Next, find your form under Form Builder > Forms. Click on it as if you were going to edit it.
In the top right corner, click on Move to bin. The form will then be archived and will now only appear under the Bin tab:
From here, you can restore it or delete it permanently (not recommended unless it was purely a test form).