question ? can we change number of hours per days ?

You cannot because some real work has been entered.

As long as parameter is defined to enter hours and som real work is entered (in hours) you cannot change the conversion unit (hours per day).
You must know that ProjeQtOr internaly stores all work data in days.
This the way to set possibility to enter real work in hours and display work on activities in days.
So imagine you have one user whow entered 7 hours on one day, with 7 hours per day defines.
If you changed conversion value to 8 hours per day the user returning to real work allocation screen would see 8 hours... and would not understand where this extra hour come from.

On the other hand it is not possible to define the number of hours per day
For a team or by resource?

NO.

An 8h day goes to all users
But if we have BOB working it at half-temp is 4h?

Define bob capacity of 0.5

It is defined on Resource.