I will use ethereum as private blockchain. Then I'd like to change the confirmation time from 15 seconds to 1 second. Can I change it? Could you tell me if you know that?
No, you can only control the confirmation time through the difficulty of the blockchain which in turn depends on the mining resources allocated for the respective private net.
You can set a "small" difficulty in the genesis block and see the average confirmation time for a block (you'll have to run a few tests with the miner of your choice). Adjust the hashrate/number of mining threads in the client to adjust the difficulty.
Note that in current implementations setting the difficulty lower than 1024 will prevent any later increase regardless of the hash rate. See the yellow paper (section 11.5).