Why is my CRUD application's update function not working in Laravel?

I encountered difficulty with the update function. Even after the data has been updated, it does not change. Is there something wrong with my code?

Controller

public function update(Request $request, Mahasiswa $mahasiswa)
{
    $validatedata = $request->validate([
        'nim'           => 'required|size:8|unique:mahasiswas,nim,'.$mahasiswa->id,
        'Nama'          => 'required|min:3|max:50',
        'jenis_kelamin' => 'required|in:P,L',
        'jurusan'       => 'required',
        'alamat'        => '',
    ]);

Mahasiswa::where('id',$mahasiswa->id)->update($validateData);

  //$mahasiswa->update($validatedata);

 return redirect()->route('mahasiswas.show',['mahasiswa' => $mahasiswa->id])
 ->with('pesan',"Update data {$validatedata['nama']} berhasil");
}

Model

namespace AppModels;

use IlluminateDatabaseEloquentFactoriesHasFactory;
use IlluminateDatabaseEloquentModel;

class mahasiswa extends Model
{
    use HasFactory;
    protected $guarded = [];
}

Migration

public function up()
{
    Schema::create('mahasiswas', function (Blueprint $table) {
        $table->id();
        $table->char('nim', 8)->unique();
        $table->string('nama');
        $table->char('jk',1);
        $table->string('jurusan');
        $table->text('alamat')->nullable();
        $table->timestamps();
    });
}