You can not directly change the period in Material master .
The MM period you maintain in OMSY will appear as Current period in MMD.
For that MM period you need to Close the Current period & Open the next Period as per the requirement for that you need to do Through MMPV trxn..as mentioned below.
First check the Current period in OMSY trx...for your Comp code..
Then go to MMPV and enter Comp code, Period & Year
Ex: if the current period in OMSY for your Comp code is 01,2008
then you need to enter period 02, 2008 in MMPV trx...
so, that period 01 will be closed & Period 02 will be opened automatically...
You need to close periods one by one like this..
until the period you want use...
Remember you can not Roll back the closed period
Be careful in closing the MM period in PRD client.
In material master ,I mean in MM02 or MM03, you can not open the period.
1st check, which period is open in MM side in T.Code: MMRV and must ensure that all the transactions related to the periods (current and current minus one, If back posting set in MMRV) are posted. Now you can close that period /open a new one in MMPV.
For initial opening you can use MMPI and also check OMSY setting.