How to find the secondary inductance value in flyback transformer?

The square of the ratio of the turns ratio works irrespective of how you drive the transformer. So, if primary is 236 µH and secondary winding has half the turns of the primary then the secondary inductance is 59 µH.

The primary and secondary current slopes will only be equal when using a 1:1 transformer and input and output DC voltages are identical. The slope determines the voltage i.e.: -

$$V_{OUT} = Lcdot dfrac{di}{dt} = 59 text{ µH} times 847458 text{ A/s} = 50 text{ volts}$$

And

$$V_{IN} = Lcdotdfrac{di}{dt} = 236 text{ µH} times 423729 text{ A/s} = 100 text{ volts}$$

Normally, Current changes (ΔI) on primary and secondary must be equal.

That is true for a 1:1 transformer but not an N:1 transformer where N isn't 1.

If current changes is not equal, how can I calculate current change for secondary?

If the transformer steps down 2:1 then, because ampere-turns have to be equal on primary and secondary, the secondary current must be twice the primary current. Ampere-turns are transferred from primary to secondary at the moment the primary is disconnected so this is a valid approach.