How do I modify Chromium's default data directory?

I believe I somehow messed up my user profile directories with my Chromium installation. Every time I launch Chromium, it tells me:

Chromium cannot read and write to its data directory:

C:UsersMarkAppDataLocalChromiumUser Data

It lets me choose another directory, and if I choose C:UsersMarkAppDataLocalChromium, then it launches fine. I can also add the command line argument --user-data-dir to point to the directory.

However, how can I modify the default (currently C:UsersMarkAppDataLocalChromiumUser Data) data directory to change it to the correct one? I don’t only get this error when opening Chromium, but also when an application tries open my default browser.