What is correct ram for Zenbook ux305f

[g.shorto]

I need to replace my Zenbook's ram. What is a good choice?

 

[rgd1101]

http://www.crucial.com/usa/en/compatible-upgrade-for/ASUS/zenbook-ux305fa
http://www.myfixguide.com/manual/asus-zenbook-ux305fa-disassembly/
it soldered, no upgrade/replace

 

[g.shorto]

http://www.crucial.com/usa/en/compatible-upgrade-for/ASUS/zenbook-ux305fa
http://www.myfixguide.com/manual/asus-zenbook-ux305fa-disassembly/
it soldered, no upgrade/replace

Why shouldn't I unsolder it, and solder a new stick in?