options filestat;
filemane pdslib  'a.pds.library' disp=old;
data _null_; 
   infile pdslib(pdsmem) sharebufs; 
   file   pdslib(pdsmem);
   input @1 var1 $char3.; 
   if var1='efg';
     put 'xyz'; 
run;

filename pdslib  'a.pds.library' disp=old;  
options filestat;                             
data recs;                                    
   infile pdslib(pdsmem);                     
   input;                                     
   rec=_infile_;                              
run;                                          

data _null_;                                  
   set recs;                                  
   file   pdslib(pdsmem);                     
   oldlbl=substr(rec,1,3);                    
   chglbl='egf';                                 
   newlbl='xyz';                              
   if oldlbl eq chglbl then do;                  
     newrec=rec;                              
     substr(newrec,1,3)=newlbl;               
     put newrec;                              
   end;                                       
   else do;                                   
     put rec;                                 
   end;
run;