Go to Admin > Students > Group Updater - select the Current Year Group of the students.
Under Current Form Group select one of the forms that are merging and change the student attribute to current form group and choose.
You will then be able to select the other form group from the dropdown next to check all and click check all and update at the bottom. This will put all students into the same form group.
Once complete you can remove the other group from the system via Admin > Config > Core Set Up > Forms and click delete next to the form name or if you want to change the name of the newly amalgamated form you can also do this here by clicking onto edit. Please ensure that update students in group is ticked before clicking onto update.