How to increase torque and decrease RPM of a DC motor?
I shall make the assumption the machine you have is a brushedDC machine.

Torque ~= current  (Kt being the constant of proportionality)

Speed ~= voltage.  (Ke being the constant of proportionality)

In actual fact this is loosely accurate for all machine types but rather than being directly proportional it becomes a bit more complex &amp; usually involves frequency.
( see How do DC motors work with respect to current, and what consequence is the current through them?)

To reduce the speed the simplest method is to reduce the voltage be it by replacing the 9V battery by a... 8V battery or via PWM.

To increase the torque ... that a bit harder. All you can do, electrically (as you can do it mechanically via gears), is to reduce the source impedance to maximise the current that can be delivered for a given stator impedance.