how to reduce the current without changing voltage for charging a 9 v battery
Your AC adapter is rated at 210mA, but it will try to deliver whatever current the load demands. Rechargeable batteries will take very high current if you let them, so some form of current limiting is essential. You may be able to regulate the charging current with a resistor. However whether it will work and what resistance value you need depends on the specifics of your AC adapter.

A typical '9V' rechargeable PP9 battery consists of 7 NiMH cells in series. These have a nominal voltage 1.2V each, so the whole battery is actually only 1.2 x 7 = 8.4V. However when charging the voltage will go up to ~10V (1.4V per cell). 



When using a resistor to regulate current the power supply must put out a higher voltage than the battery, which the resistor then drops down at a current determined by its value and the voltage drop across it. If your adapter's output is regulated to exactly 9V then its voltage is too low and the resistor won't work. However if it is unregulated then it may put out 12V or more at low current, which the resistor can then drop down to the 9~10V that the battery needs.

So what you need to do is measure the voltage coming out of your AC adapter, then calculate the resistor value required based on the difference between that and the battery voltage. For example from 12V to 9V the difference is 3V, so to keep the charging current below 20mA the resistance required would be 3V/20mA = 150&ohm;. 

As the battery charges up its voltage increases, so the voltage drop and charging current reduces. At full charge the battery reaches ~10V for a difference of only 2V (with a 12V supply), so the charging current would drop to ~2V/150&ohm; = 13mA. This is good because it is kinder on the battery than continuing to charge it at the maximum rate.