How to change default value for a field on a form?

Override the target field and change PXDefault attribute:
        #region CaseClassID
        public abstract class caseClassID : PX.Data.BQL.BqlString.Field<caseClassID> { }

        [PXDBString(10, IsUnicode = true, InputMask = ">aaaaaaaaaa")]
        [PXDefault(typeof(Search<CRSetup.defaultCaseClassID>))]
        [PXUIField(DisplayName = "Class ID")]
        [PXSelector(typeof(CRCaseClass.caseClassID), 
            DescriptionField = typeof(CRCaseClass.description), 
            CacheGlobal = true)]
        [PXMassUpdatableField]
        public virtual String CaseClassID { get; set; }
        #endregion

Or create a graph extension, then use FieldDefaulting event and NewValue property:
public class CRCaseMaintExt : PXGraphExtension<PX.Objects.CR.CRCaseMaint>
{
    public virtual void CRCase_CaseClassID_FieldDefaulting(PXCache sender, PXFieldDefaultingEventArgs e)
    {
        e.NewValue = "DefaultValue";
    }
}