Can I reuse last year data?

Yes, you can. In fact it is quite simple:

Open your last year timetable file (e.g. Timetable 2019-2020.roz).
Use menu "File - Save as" and save this file under a new different name (e.g. "Timetable 2020-2021.roz").
Then you can add new data and remove data that is not valid for the upcoming school year.

Note: In some cases it makes sense to rename the classes. For example, if your new class 6.A will have mostly the same lessons as it had in the last year, you can simply go to menu Specification - Classes, Edit the class 5.A and change the name to 6.A. By doing this, your new 6.A will have the same lessons as it had in the previous school year when it was called 5.A. So all you need is just to edit the counts of the lessons, or particular teachers and classrooms. No need to enter all lessons again from the scratch.
You can save a lot of time by reusing already entered scheduling data.