This looks like the Jovikids Ranger, is that right?
I agree with others that you need to move the straps/headrest up, usually they are attached so move together. The manual says that straps should be level with the child's shoulders. I'd probably bring the seat inside so you can fiddle with it and try out different settings without her getting cold.
The insert looks like it's all one piece according to the manual and should be used up to 75cm, which is around 9-12 months old, LO looks much younger so I think the insert should stay in.
Often with cheaper seats the inserts are badly designed, so they move around all over the place and you end up having to hold the insert and the straps and the child in place. It's a huge pain. I have got a spare Cozy n Safe one for travel which is like that. You will probably want to rotate the seat to the side and then loosen the straps fully, place child in in a lying position, usually the weight of the child moves the insert down, so put your arms under her one each side and sort of shift her up until the insert is sitting with the gap for the straps in the right place. Fasten the straps, then pull them up through the buckle so it's snug at her waist/tops of legs, hold with left hand, use right hand to check everything is OK with the shoulder strap position and the insert hasn't shifted, then tighten bit by bit with the right hand checking every so often that the straps are in the right place.
You will have a frustrating time when she learns to sit up (if not already) because they tend to lean forwards and try to grab the buckle. Bring a soft toy or rustly toy and once you've tightened the leg straps, offer this above her head and sort of lead her back to a lying position before you tighten the shoulder straps. But this will only work if the headrest and straps are adjusted properly.
Straps need to be tight enough you can't pinch any loose webbing. The instructions aren't clear on this IMO.
Honestly, I am not the world's biggest fan of this brand, I think their instructions are appalling and it makes the seats really difficult to use, I am a bit dubious about the quality of them as well. If you got it on amazon recently, I would look into whether you can return it and swap it for something else, e.g. this one: https://www.amazon.co.uk/Joie-Spin-360-Car-Seat/dp/B07PZFBXCN/
This video is a different brand but shows how to tighten the harness in the way I explained in case the explanation is confusing.