Getting the router's config
Looks like Huawei uses the same encryption key on the configs for all routers and according to a Spanish language forum it was present in a file on the router /etc/wap/aes_string.
I have found the source code for the tool aescrypt2_huawei that is provided and a walk-through provided on a blog called TheZedt. It seems the Huawei uses the same configuration encryption key
hex:13395537D2730554A176799F6D56A239 on all of its devices and the only thing special about the binary version of the tool is the fact it adds an 8-byte header to the file. You can see the source code link for a more detailed explanation.
As the walkthrough and @Blz mentions you can modify the
0 and reupload it to the router, but... I strongly recommend you add a new user rather than modify an existing account. You'll need to generate a new password hash using the formula SHA256(MD5(new password)). Test to make sure you can generate the same hash as the root account in the file by testing with 'admin'.
On a Mac
echo -n admin | md5 | tr -d '\n' | shasum -a 256
in the file
<X_HW_WebUserInfoInstance InstanceID="1" UserName="root" Password="465c194afb65670f38322df087f0a9bb225cc257e43eb4ac5a0c98ef5b3173ac" UserLevel="1" Enable="1" ModifyPasswordFlag="0" PassMode="2" />