During online application form entries, it is common to make such mistakes. This is an unintentional and simple mistake, I do not think that we need to worry unless it is a major mistake in age, education and cut off marks related to eligibility criteria.
Most on line form submissions will NOT allow you to edit once you pay the fees or the click on the final submit button.You can give it a try.
Please check the application filling guidance notes or the advert if there is a mention of any contact details for any errors related to the application form. It would be an email or a postal address.
If not, please check for the contact details of the recruitment authority.
For instance, I found one address for contact details for university non-teaching staff recruitments.
Central Receiving Section
Office of the registrar
University of Calcutta
87/12, College Street
You can send an email or write to the relevant recruitment in-charge, clearly stating your error and keep the acknowledgment with you in case, they ask you while you reach the document verification process.
If you are communicating, please do not forget to mention clearly the 'Job title", 'Job advert date and announcement number/date' so that it would be easy for them to identity and process it.