It's supposed to be "final," but, if you made a mistake, given that it's only 5/1, I'd send them an e-mail explaining the snafu and ask to change it PTE (hopefully to the same school?).
It's difficult to see why they wouldn't understand given how early it is, but, it's really up to the school at this point. There is no rule governing this, other than the one that says CTE is final and you are supposed to withdraw from all other As and WLs. The repercussion could be that they rescind the A, so you really want to clear whatever you are going to do with the school.
You might be able to make the change and have them not even know, but I have no way to know what kind of audit trail the system creates, or if they receive some type of automatic notification when you CTE, so it seems risky to just make the change on the down low and hope for the best.
Do you intend to PTE at the same school? That would probably be easier than using this to switch schools, because the other school might possibly have already rescinded you since you wouldn't have selected PTE yesterday, adding yet another wrinkle. The fact that this is a weekend could make this moot, since nobody could really be hurt by you making a change so soon, but, again, it depends on whether the school is willing to overlook a simple mistake or if they want to make a thing out of this. JMHO, but I'd reach out to them before doing anything.