SoftwareSerial

I read that later UNO3s have SoftwareSerial on their chip. A sketch I am attempting to run includes #include <SoftwareSerial.h>.
How can I access this library to edit _SS_MAX_RX_BUFF please?
Once I have edited the memory will it save to the UNO and remain at that value every time I start up the sketch?
Any help would be appreciated.

Where did you read that ?

RayRogers:
I read that later UNO3s have SoftwareSerial on their chip. A sketch I am attempting to run includes #include <SoftwareSerial.h>.
How can I access this library to edit _SS_MAX_RX_BUFF please?
Once I have edited the memory will it save to the UNO and remain at that value every time I start up the sketch?
Any help would be appreciated.

Read where? Any arduino chip has only bootloader code installed in it. To use Software serial is a matter of including the library in your sketch and then using it's functions properly.

RayRogers:
I read that later UNO3s have SoftwareSerial on their chip.

You have misunderstood or misremembered.

UNOs do not have SoftwareSerial 'on their chip'. SoftwareSerial is one of many libraries contained in the Arduino IDE. In recent versions of the IDE it is a standard library; in older versions it was an external library which would have to be downloaded and installed separately; perhaps that's what you're thinking of.

The SoftwareSerial library is provided in source code form. and if you want to change the behaviour or change any configuration constants you could edit the source files in your installed copy. Any sketches you built subsequently based on your modified copy of the library would include your changes.