Where you are trying to change the scan schedule in SEPM or in client side?
If a scan is configured in SEPM we can change it in SEPM only
For this you can edit the antivirus and antispyware policy
Come to administrator defined scans select the scan which you want to change, click on edit go to the schedule tab here you will get an option for changing the timing
Also ensure that the user which u logged in to SEPM having the right to change those settings....