You have 3200 MHz, as I can see in the screens you posted and in Dell's memory specifications for your laptop. You may add another DDR4-3200 stick. However, sometimes mismatched RAM brands/models may not play well together. If that happens, you can try another brand. If it still doesn't work, you may have to remove your RAM stick and buy a matched pair.  I don't know your current RAM brand, but it uses Micron chips (many brands use Micron chips). Crucial uses Micron chips (both brands are part of the same company), so you could try Crucial. Follow the disassembly and reassembly steps in Dell's Service Manual. Here are the instructions for removing RAM.Look on YouTube for videos showing disassembly of your model. But, be cautious with them, as some use steps or methods that are not appropriate. Use the videos to add to your understanding of what to do, but follow the steps in Dell's manual. Whenever touching components or working inside a computer, wear a grounded wrist strap, also called anti-static wrist strap, ESD wrist strap, or ground bracelet. It's a cheap and sensible precaution. Rest the laptop on an anti-static mat or at least a reasonable alternative such as corrugated cardboard.
I want to upgrade my Dell Vostro 5402 8GB ram to 16 GB. What should I look for another memory stick? Can I get other company than Dell (for example: Transcend, Kingston, etc.) ? How can I know which stick do I have without opening the laptop?I was offered these [two sticks][1], I don't know which one to choose: [URL] This is my laptop specifications: [CPU-Z memory tab][2]: [URL] [IMAGE]   [SPD Tab][2]: [URL] [IMAGE]   [1]: [URL] [URL] [URL] [URL]
Upgrading my dell vostro 5402 8 GB ram to 16 GB