How can I tell what changes were made in the isCOBOL latest update?

Each installation setup includes a README. This README file lists all the new features in that release.

There is also a file named CHANGELOG.txt that lists all changes in the release, in order of build and ID number.
For instance, if you update from version 2023R1 build 1090.6 to 1090.7, the CHANGELOG lists these changes:

### Build 1090.7 (09 Mar 2023)
    = [ID #29684] C$KEYMAP and C$ENVMAP routines are now more relaxed on parameters check
    = [ID #29677] fixed a wrong stream generated by JSONStream class with nested dynamic occurs
    = [ID #29661] fixed an issue on puttons with bitmap-scale set and bitmap-width not set
    + [ID #29650] JDBC driver version is now traced in the log
    + [ID #29649] new compiler option -ccmf to have MF compatibility in numeric literals passed in CALL statement
    = [ID #29625] fixed a wrong arg-type returned by C$CARG and C$DARG routines in a particular case
    = [ID #29489] fixed an issue on C$OPENSAVEBOX under WebClient with Cluster environment

You can install each release over the last one. There's more information about that in KB #330