The opening stock figure is pre-populated because one of the benefits of the online form is that HM Revenue and Customs (HMRC) can carry forward information from previously submitted Declarations. If the opening stock figure is pre-populated you'll not be able to change it, as this will be the 'closing stock' declared in the last period. If you have made an error on the last Declaration then you must use the 'Adjustment' and 'Reason for Adjustment' fields to advise us of this error.
If your opening stock has not been pre-populated this indicates you've not submitted the last Declaration and you are submitting your W1 Return out of sequence. Please make sure that all Declarations are filed in sequence, so that the opening stock is always correct on your W1 Return. Submitting your Declaration in sequence will also avoid you receiving warnings and/or penalties if you submit Declarations after the date they are due.