To update Linux Mint from the command line, use these two commands.
sudo apt-get update
This, I think, refreshes your system's list of what updates are available.
This actually replaces what's on your system with newer versions. Mint (I think) does dependency checking automatically, so that if the new software needs new parts underlying it, Mint will install those automatically as well.
The second command will present you with a list of what's about to be replaced. You'll have to OK the replacement before it starts.
Of course, there is probably a graphical way to update, too.
The most straightforward way to edit PDF files on linux that I found is to use LibreOffice to open the PDF, make your modifications then export it back to PDF.
However you have to understand that PDF is actually a complex/messy standard and that you will need to do things in a specific way when editing it if you want your generated PDF to be properly displayed.
Also make sure that the exported PDF has the same size as the imported one, since there is a difference between the european A4 format and US letter format which could totally break the layout.