When i use official nextion and want to make some test on a preexisting
HMI sketch,it changes the original without asking me,so i have to always
make a copy of the original before i play with.
Can’t see where i could uncheck this autosave,how can i? 
It’s not auto save like in Word or Excel where the work in progress is saved in fixed time intervals, for example every 30 seconds.
Automatic saving happens always in the Nextion Editor when the .hmi is compiled, either by clicking the compile button, the debug button, or the upload button. This is to keep the uncompiled and the compiled versions in sync / coherent, and can not be disabled.
But I agree with Coconut.
I have a working program, but want to try something and it fails, then I can decide not to save the program so that I can reload the working original, but the Nextion IDE does not give this choice to the user.
(Undo has its limitations, does not work 100%)
Now I always first have to make a copy of the working original (or save it under a different name) to make sure that the working program is preserved.
Use Git. This allows always to revert everything to The last commit with a single mouse click.
In the past,many of the softwares had init files like .ini…xml to change
settings,so maybe i can desactivate the autosaving,where is the nextion init file? syscom.bin?