if the name of the file is mylog
then use this command at the command line
or if you want to view the last 15 lines,
if you want to edit the file then for example last 30 lines
the above will give you the number of lines
let's say there are 10,000 lines
then do some simple math and you will only have to edit the last 30 lines.
head -9970 mylog > upperfile;tail -30 mylog > lowerfile;pico lowerfile
will open the pico editor and let you edit the last 30 lines of the file
when you are finished with editing the file with the pico editor(who uses vi editor nowadays?) save and exit by pressing ctrl x
then at the commandline
cat upperfile lowerfile > mylog
will give you a new mylog with the last 30 lines modified to your taste and needs.