There are different computer programs that different places use for their MDS computer input. So, I'd have no idea how to fix this error in your program, not knowing what program you're using.
You also didn't say whether or not the error was already transmitted. That makes a little difference. If you haven't transmitted it yet, then it can be fixed. Just ask your trainer how on Tuesday.
If it was already transmitted, don't sweat the small stuff. There's another special transmittal that you send when you mess up. No one likes to send the correction, but it happens often enough that there's no need to worry about it.
As for the 5 days, don't worry about that, either. There are grace periods for every one of them. If you get it corrected by next Friday, you'll probably still be within your grace period, and no penalties.
If you have the error, but have not yet transmitted, I'd wait to fix the correction on Tuesday with some help instead of needing to send the correction report.
Relax. You can do those MDS's for years and still learn new stuff about them, and make boo-boos. You're doing fine. There's a lot to learn with those things!
By the way, I suspect your NM told you to play around with it over the weekend because she knew she could fix anything you messed up before it was actually transmitted!