My gopro hero 3 had a password set I couldn't remember. It seems changing it isn't that straightforward, most guides on the internet redirect you to the gopro site, where you have to enter your serial number to obtain an update.zip file, which contains 2 textfiles. Here is a guide to do it without going trough their site.
Get an empty microSD card (or format it) and add the following 2 files to the root directory of the microSD card. Change WIFI_SSID to your own SSID, and change WIFI_PASSWORD to the password you want to use.