How to change Valuation Class in Material Master

Option1:
Goto TCode MMAM - This TCode is for changing the material type and not valuation class.

If you change the material type in MMAM transaction automatically, the valuation class will be changed in material master. The system will take the changed material type valuation class that was created for that material type.
Option2:
Goto TCode MM02 - Accounting View 1 - This TCode is to change the valuation class directly.
Whether you can or cannot change the valuation class directly without changing the material type depends on your own company img settings.

If you want to change the valuation class, there may be because of one or more of the following already exist for the material:
1) Valuated stocks in the current period or in the previous period.
2) Open purchase orders may be there.
3) Production orders which a goods movement has already taken place.

For this first you have to do the following:
 If valuated stocks already exist, you can change the valuation class only:
a) Post the stocks of the material to an interim account.
b) Change the valuation class in the material master record.
c) Post the stocks of the material back to their original account.

 If open purchase orders already exist, you can only change the valuation class if you first flag the corresponding purchase order items for deletion.

If production orders exist for which a goods movement has already taken place, you can only change the valuation class if you first set the status of the production orders to deleted
By this you can keep the same material type as FG and can assign the proper valuation class.