Installing 2.1 beta 3 stuck at step2

[JonasTheGamer]

Hello everyone,
I get stuck at step 2: Writable Check

I filled in my ftp username and password and i told to install at /forum
Then i got this:
"Some files need to be writable for SMF to work properly. This step allows you to let the installer make them writable for you. However, in some cases it won't work - in that case, please make the following files 777 (writable, 755 on some hosts):
Settings_bak.php"

I'm litterally a noob in that. How can i do that?
Thanks,
Jonas

[JonasTheGamer]

Aha,
I callled the file Settings_back.php instead of Settings_bak.php 
Marked as solved!