Its probably zipped, so you need to unzip it.
Then patchadd 118855-19 from the directory where you unzipped it.
You may need to download and install prerequisites first.
You might be better off downloading the recommended patch cluster which will include that plus all prerequisites amongst others.
Since thats a kernel patch, you should do it with the box quiescent ie no programs running, nobody (except you) logged in.
Sun actually recommends doing it from single user, but quiescent is usually good enough