Luciana,
use a voltage divider. The simples form is build with two resistors. In your case two resistors of the (exactly) same value.
Connection is (Signal)-(Resistor1)-(ADCinput)-(Resistor2)-(SignalGround)
Depending on your DC Signal source and the input resistant of your ADC the value of your resistors should be in a range of from 10k up to 1M.
47k resistors are a good starting point
🙂 Since you said it is DC you don't have to care about frequency compensation.
Error calculation will be your homework
Greetings from Germany
Henrik
LV since v3.1
“ground” is a convenient fantasy
'˙˙˙˙uıɐƃɐ lɐıp puɐ °06 ǝuoɥd ɹnoʎ uɹnʇ ǝsɐǝld 'ʎɹɐuıƃɐɯı sı pǝlɐıp ǝʌɐɥ noʎ ɹǝqɯnu ǝɥʇ'