How to change model default values ?

I am able to set the qpos initial values at default pose. Due the change is qpos0 values, the robot becomes unstable(because of high qvel values) when mj_step is performed. Interesting thing is that the robot is stable when all the qpos0 values are zero. For non zero qpos0 values, there is high change in angle, hence high force acting which make the robot unstable.

How to set the values at default pose, without any change in qvel ? Since we are dealing with humanoids, stability is a major criteria.

I hope you understand. Below is the code.
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for(int i=0;i<13;i++)
{
m->qpos0[13+i] =newqposvalues;
}
mj_resetData(m,d);
mj_kinematics(m,d);
mj_step(m,d);