Here is the command and a JCL to change the GDG limit. It doesn’t matters what was the earlier limit, it will change to the limit specified now.
From any command line:
TSO ALTER 'YOUR.FILE.NAME' LIMIT (30)
or from Option 6:
ALTER 'YOUR.FILE.NAME LIMIT' (30)
Run an IDCAMS job:
(Supply your own JOB card)
//IDCAMS EXEC PGM=IDCAMS
//SYSPRINT DD SYSOUT=*
//SYSIN DD *
ALTER YOUR.FILE.NAME LIMIT (30)
(Be sure there is a space before the ALTER in an IDCAMS job)
Also to know more about the ALTER command, please refer the below site. It has many functions apart from limit change for GDG.