I updated the DFHSIT$$ in CICSTS54.CICS.SDFHSAMP where I changed SPOOL=NO to SPOOL=YES. How can I make CICS to load the updated table?
Now that you have updated your SIT table in DFHSIT$$ you will need to compile it, see https://www.ibm.com/docs/en/cics-ts/5.4?topic=parameters-assembling-sit for details.
Once the compilation has been done you will need to restart your CICS region for it to pick up the changes. I assume that you already have SIT=$$ specified within the SIT overrides in the CICS JCL, for example:
//SYSIN DD *
SIT=$$
.
.
.
/*