How do I change PGA gain in this Arduino program?
The symbol PGA appears in exapmle.ino (sic) twice, once each in lines 28 and 30:

 #define PGA 1                     // Programmable Gain = 1
 ...
 #define VFSR VREF/PGA


It is not used elsewhere in the example or the library code.  The only effect of changing its value from 1 to 32 is to change the value of VFSR, and asVFSR appears only in line 30, that effect is nil.

Looking at the ads1262.cpp file from github, it appears that ads1262::ads1262_Init() initializes register MODE2 to 0x6, via:

ads1262_Reg_Write(MODE2, 0x06);


which per the ADS1262 spec sheet, page 93, table 44, enables the PGA and sets its gain to 1 and sets the sample rate to 60 SPS.

Apparently, if you wish to use some other gain setting, you should define PGA appropriately and put a corresponding setting into MODE2.  For example, if you want to set up a gain of 16 and sample rate of 400 SPS, in setup() you could say

PC_ADS1262.ads1262_Reg_Write(MODE2, 4&lt;&lt;4 | 8); // 4=gain 16, 8=400 SPS


More generally, you could set up some constants for gain codes and SPS rates, and reference those constants when writing to MODE2.  For example, after

enum { GainCode1, GainCode2, GainCode4, GainCode8, GainCode16, GainCode32 };
enum { SPSCode2p5, SPSCode5, SPSCode10, SPSCode16p6, SPSCode20, SPSCode50,
       SPSCode60, SPSCode100, SPSCode400, SPSCode1200, SPSCode2400,
       SPSCode4800, SPSCode7200, SPSCode14400, SPSCode19200, SPSCode38400};


you would say

PC_ADS1262.ads1262_Reg_Write(MODE2, GainCode32&lt;&lt;4 | SPSCode2400);


to set the gain at 32 and the sample rate at 2400 SPS.

(Note, enum is a C language element used to define integer constants.  In an enum, each symbol mentioned without an explicit value has a value one more than the previous symbol, or zero if there's no preceding symbol.  So GainCode1 is 0, GainCode2 is 1, GainCode4 is 2, etc.)

Note, to get better ultimate resolution, you may need to combine multiple readings, or may need to set the ADS1262's filtering parameters.