U can buy this adapter to refill sodastream tanks from ur 5lb tank.
https://www.amazon.com/gp/product/B0BNNXXG6C/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&th=1
Video: https://youtu.be/BeZmesxhk8Y
Or if u want a direct connect , u can do this
Video https://youtu.be/rBqSxvr56s4
If u have a terra sodastream, then u need the quick connect adapter (and the quick connect unscrewed from any empty sodastream tank)
I bought that adapter on Amazon. Used it for the first time last weekend for one of my empty pink canisters. I was a bit nervous as I'm not super handy, but it worked out great. My tank does not have a siphon, so it was awkward to fill upside down, but followed the directions carefully and it went fine. Just put it in and it works great.