To "narrow the beam" will likely require replacing the existing lens, or perhaps adding an "overlay" lens. However, if you do replace/augment the lens, it's not clear that will make the sensor less "jumpy". Here are some sources for Fresnel lenses that may help.
Before shopping for optics, you should definitely make time to read this article from Fresnel Technologies Inc.
The sensor you're using doesn't have much useful documentation. Some PIR modules have "sensitivity adjustments", but it's not clear if yours does. You may consider adopting a policy of No documentation - No sale in your buying decisions.
In any event, the output of the sensor is a single bit - it's "ON", or it's "OFF", but a sensitivity adjustment would allow you to experiment with different values. Shame on your vendor if they didn't provide this adjustment. Because devices from different vendors use different components, it's difficult to provide a pat answer to your question. See suggestion #2 below for a potential software solution to "jumpy".
You should do some research. This is an informative article; this is another informative article; this is yet another informative article. Take a look at these; edit your question if you wish.
One general technique to address the "jumpy" output is integration over time. You can do this in software on your RPi. You will have to learn to discriminate between "false alarms" and "valid detection" events. Integrating the PIR output over time is a place to start - simply put, count the time the sensor output is asserted during a "detection interval", and make a "valid detection" decision based on that time.
Finally, as one peruses the vast sea of information available on the subject of PIR sensors, one item that caught my eye was the use dual sensors. In some cases, these involved two or more PIR sensors with different fields of view. In other cases, PIR was combined with other technologies (e.g. ultrasonics) in a sensor fusion approach. It seems that between the software and hardware options, there are an almost infinite number of potential solutions.