Replacing brake fluid is really no different from most other cars. I did it for the first time a couple of months ago using a pressure bleeder I bought at Amazon. i3 wheels have a deep offset on their back sides making access to the bleeder valves more difficult that on our previous cars. I couldn't change the fluid with the wheels on the ground, so I lifted our i3 onto jack stands and removed each wheel. It was time to rotate the wheels side-to-side and to clean the rust dust off their back sides, so all was good.
I also checked the cabin air filters. Because our i3 has been driven only ~13k miles, much without the climate control on, and because we have very clean air in Honolulu, the air filters looked quite clean, so I kept my new filters for a later date.
These are the two things that our BMW dealer did every 2 years during scheduled maintenance visits, so that's what I did. I also inspected the suspension and brake components as the dealer would likely have done during its multi-point inspection. I discovered that the dust covers over the shock absorbers were beginning to tear as many others have reported. The suspension bump stops were also decomposing but still functional. I don't plan to replace the dust covers or bump stops at this time.
YouTube has videos that show how to replace i3 brake fluid and cabin air filters, so that's a good source of information. realOEM has parts diagrams that could help.