it is impossible to update software to a new OS cause it takes days to download 12 Gb, it starts each time from the beginning!!!

There are factors:

1 - Like poor Wifi connection

2 - Slow wifi connection

3 - Too many other people downloading at the same time and Apple Severs are overloaded

4 - Software on the computer interfering with the download process including AntiVirus Software

5 - Running behind a VPN which may increase Packet losses

6 - If via wifi too many Other devices using the wifi at the same time reducing the Bandwidth to your computer. 

7 - Best Results are via Ethernet Cable to Router

Can you set your Mac to not sleep?

Try connected via ethernet or to another network. It only takes me less than 1 hour to download via wifi.

disable the sleep mode, poor internet speed suppose worth a check, my advise don't upgrade first, wait till it's stable, done a few weird things to mine recently, I should have waited.

you could try going into SystemPreferences > EnergySaver and see if "Power Nap" is enabled. that setting should allow the mac to update while sleeping.