Wen I use it to measure capacitors it do not do it's 0 before measuring,
even without any wires plug to it there is alway a .1 nf reading, there is
only two trimpots on the pc board, how can we reajust to have 0 ?

On the lower capacitance scales, it _shouldn't_ be zero. The test leads have
a certain amount of capacitance.

If you've removed the leads, a residual reading might be due to internal
capacitance, or the converter's LSB flopping.

In any case, if you're reading microfarads, 0.1nF (100pF) is hardly of any
concern.