I'm so proud, I have to brag to someone! Hopefully I can help someone too. First, my backgroun: I'm a 49-year-old mom. I drive a Subaru station wagon, for God's sake. I have no tech background, but do admit to a DIY gene I got from my dad.
Anyway, both of my kids' iPhone 6 phones started getting little to no wifi connection. They could maybe get three bars directly next to the router, but as soon as they moved away, they'd lose it. I bought a GPS Antenna Signal Flex Cable Ribbon Replacement Part for IPhone 6 (4.7'') for $4.90 (actually, I bought two, hoping to fix both phones...glad I did, because one came defective and I didn't look to find out until I had the phone completely open and dissected. Maybe order two, since they're cheap, just in case) and a $7.99 tool kit off Amazon, and fixed it!! I followed two different youtube videos, but ultimately only needed this one...there are a bunch, though. The phone works perfectly with a strong-as-new wifi connection!
1) The first video I used ended after removing what the instructions called "wifi receiver" and I thought, "F*&K!" thinking I had ordered the wrong part. But the second video went on to remove that part, calling it the "cover" and the "mother board" to reveal and replace the antenna, which looks sort of like an "F." This is the only part I replaced and it worked to restore the wifi signal completely.
2) There are roughly one million screws ;) to remove, and they're minuscule. I drew pictures (using #s to track the order in which I removed the parts, and labels) and also took pictures and used the "mark up" editing tool on my phone to make notes. It wasn't pretty, but there's no way I would have been able to put the phone back together had I not done that. The hardest part was removing two stripped screws...I found a youtube video with hints for how to remove those too...finally I did it with a combo of the tools I bought and tweezers (which the tool kit doesn't come with, and I recommend having). Altogether, I watched the videos on my iPad, used my phone to take pics, and used a pen and paper for notes and laying out screws by my labels. I also used a really bright bedside/task lamp, my reading glasses, and a glass of wine!
I think this is all of the hints I have. It's a little scary, and there are some connectors that are so tiny, it's amazing you can snap them back on (like the battery connector), but you just have to go for it. If your phone doesn't get wifi anyway, and you don't have unlimited data, I'd go for it! It took me two hours (would have been 1.5 if it weren't for the stripped screws). Good luck!