A motion to modify will only be considered if it meets certain criteria. There must be an increased need resulting in a difference of at least 20% versus what is currently being paid or received according to "Form 14" calculations. With that being said, contact your Circuit Clerks office for the required filing information. Start with their website. Your county may have a pre set form you can just fill in the blanks and file. Other counties, like Jefferson, do not have a set form. You have to file and then the judge determines whether or not he will accept your form as an appropriate filing. If you have any motions from any previous hearings, copy those. If not search the Circuit Clerks offices for Missouri; I am sure you will be able to find a pre set form. Again, it will be up to the judge to determine if it is acceptable. There are also many "forms" services online you can utilize for a fee.