I configured my Fritzbox router at some point, but I've completely forgotten the login details. Now I want to change the Wi-Fi password because my roommate is moving out. Can I do that without the admin credentials or do I have to reset the router?
You won't get in without the admin credentials, sorry. You can't just go into the web interface and change the Wi-Fi password without logging in first. And no, you can't access it via Wi-Fi itself either - that's exactly what the security here is supposed to protect against.
Your only option is to reset the router. With a Fritzbox, there's a small reset button (usually on the back or bottom, sometimes hidden in a recess) that you need to press with something pointed like a paperclip for about 10-15 seconds while the device is on. After that, the router restarts and all settings are gone - including the old admin credentials. Then you can log in with the default access data (usually admin/blank or something similar, check the manual or the back of the router).
Important: After the reset, you'll also lose all your other configurations - so your internet settings from your provider, any port forwards, everything is gone. After that you'll need to set up the router from scratch. It's a bit of work, but if you don't have the access credentials anymore, you don't have any other choice. This time write down the admin credentials or save them somewhere so this doesn't happen again.
Forget about the hope that you can change the password without admin access - it doesn't work because the Wi-Fi settings are stored in the Fritz!Box itself and only the administrator is allowed to access them. So you basically need either the login credentials for the web interface or you have to go the hard way.
The harder option is a factory reset, but heads up: that *really* resets everything* - not just the Wi-Fi password, but also the admin passwords and all other settings. On the back of your Fritz!Box there should be a small reset button that you need to press with a pointed object (ballpoint pen, toothpick) for about 10 seconds while the device is powered on. After that, the router restarts and you can log in with the default credentials (usually "admin" / "admin" or there's no password on the first login). Then you reconfigure everything and set a new admin password plus a new Wi-Fi password.
If that puts you off, you could also dig out the Fritz!Box manual or look up your model online - sometimes there's even a hint on the device itself. Some models also have a special function where you can re-register over the phone, but that's pretty rare. Unfortunately, the reset is the most reliable way if you really can't get in.
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