I couldn't even do anything with the Update Assisitant, I needed to reinstall my win. I didn't know I was gonna reinstall it, It just told me it would take a lot of time to update win10. Turned out it wanted to update TO win10... Or something like that. Never mind, updated to date in half or one hour so it was worth it. Nothing more to update right now.
I used the page below after searching for Update Assistant for 1903 build. I thought I found it but as link shows it's not properly what I was looking for. Didn't lose any programs or files as far as I can tell after 10 minutes of random clicking.
https://www.microsoft.com/en-gb/software-download/windows10
Posting only as a secondary solution. Thanks for the help, though. Especially to Al0923.
Edit: Looking at it again, it seems like it was the Update Assistant in the end, it just took me too long to install and it wrote out inappropriate things. Nooo, not like that. Just things that didn't belong to an Update Assistant's mouth...