Is dell inspiron 15 (5558) upgradeable from 6gb ram to 16gb?

Solution
Yes, it should support two sticks of 8GB memory. You should be okay buying any of the major brand memory like from Corsair, G.Skill, Micron, Kingston, etc. Just make sure you stick with the same speed as what Dell put in there (1600MHz DDR3). Also buy in a kit if you can instead of individually. Occasionally when running dual-channel RAM sticks from different manufacturing cycles, they don't always play nice together that can lead to random blue screen crashes.
Yes, it should support two sticks of 8GB memory. You should be okay buying any of the major brand memory like from Corsair, G.Skill, Micron, Kingston, etc. Just make sure you stick with the same speed as what Dell put in there (1600MHz DDR3). Also buy in a kit if you can instead of individually. Occasionally when running dual-channel RAM sticks from different manufacturing cycles, they don't always play nice together that can lead to random blue screen crashes.
 
Solution
One more thing: I forgot to mention there are two voltage versions of DDR3 1600, low voltage at 1.35v and regular 1.50v. I believe Dell puts in the low voltage version, which may be labeled something like DDR3L or 1600LV or even PC3L 12800 by both Dell and the OEM memory manufacturers. Also go with CL11 on the cas latency rating to be safe...I read where CL9 memory doesn't work on many Dell laptops.