It's not difficult if it is an upgradeable model. But you have to get the right parts as they use proprietary connectors. Some are highly model specific and some support a range of Macbook Air/Pro models. So make sure you get one which fits your model.
You have a few options:
OEM: These may be new or pulled from existing MBA.
Third Party: OWC (MacSales) and Transcend have options for many Mac models. Pricier than standard SSD but will be guaranteed for your model.
Adapters: These are the most technical. The advantages being you can choose from a wider range of SSD and save money. The disadvantage being difficulty. You must know what your machine requires in an adapter and SSD. Older models require PCIe SSD newer require NVMe SSD. Also make sure you don't accidentally buy an adapter which lets you install an MBA SSD in a PC. As there are Apple to M.2 and M.2 to Apple adapters. Check customer reviews too as there are a lot of junk models.
The 2018 model is soldered. It will never be upgradeable. Just as the Macbook and Macbook Pro. The Macbook Air was just spared longer as Apple didn't make this change until 2016. Although there is a 2017 MBA. It is not really a 2017 model. It is the same model as the 2015. I think all they did was bump the CPU up one level (same generation) in the base configs then slapped the new model label on it.