when you open the wallet, side by side with the staking button there's a precomputing button, just click the grey area and it'll become blue.
enabled!
Ah.. Thank you very much may be this will bring me some luck in the staking.
Guys, when I download new update, all synchronized datas will remain? Because after few days I am still on 30% due to my slow internet connection.

Yes, all the synchronized data should remain intact. I would still suggest you to download the chain snapshots because they are much faster to download.
yeah,ok. Downloading every time a new snapshot to enter your wallet is somewhat tiring, as for me...