Pedro is absolutely correct, but it's safer to close/switch the fw.log (active log-file) 1st, unless that somehow troubles you?
that is done automatically when it reaches the max size of 2GB or daily (>=R80) or GUI configured.
so run this:
fw logswitch <chosen_name>
# if you don't name, then it names it by default as the current date/time (2020-04-26_091200.log).
Copy (all <name>.log*), via scp/winscp or any other way. Put in $FWDIR/log/ & open it via Tracker/Open Log-File.
If you've already renamed it, then best run the fw repairlog <new-name.log> & verify it succeeds, then copy all <name>.log* files.