It would help if you would provide a bit more concrete info on what you really want to rename rather than such an abstract example, but the basics work like below (assuming you want to rename certain specific host values and not generically want to rename all host values for a certain source/sourcetype).
[host::a] TRANSFORMS-rename_host = rename_host_a
[rename_host_a] REGEX = . FORMAT = host::b DEST_KEY = MetaData:Host
For syslog, this is generally done with
indexes.conf. You must configure
syslog to write the originating host as part of the path to the files.