Field is not writeable: CronTrigger

Hello,

I'm trying to do the upgrade of the PreviousFireTime field into CronTrigger, but I try an error:

Error: Compile Error: Field is not writeable: CronTrigger.CronExpression
LINE: ct.PreviousFireTime = VariablePreviousExec;

Why?

....

 CronTrigger[] acctsList = [SELECT PreviousFireTime FROM CronTrigger where CronJobDetail.Name =: VariableName ];
        for (CronTrigger ct : acctsList) {
            ct.PreviousFireTime = VariablePreviousExec;
        }
        
        try {
            upsert acctsList;
        } catch (DmlException e) {
            // code...
        }

.....

Thank you!!!