Firstly, do not use square logos. Rectangular logos are better. You can design a logo in the center of a square then crop it.
Some samplease are shown below:
Steps for changing your logo: 1. Login to your admin section 2. Go to GENERAL SETTINGS 3. You will see the option for Logo. Click Browse then upload your logo 4. Click UPDATE
You can use same method for DASHBOARD LOGO which must also be rectangular.