To change the logo in our joomla templates, there are two ways to do that.
1- In joomla backend go in extensions - templates and click on the template. In template parameters - logo tab for "Image default or Upload" select Upload Logo. In "Upload Logo Image" browse and select your logo. In some templates you can add the width and height of your logo. Click save button.
2- Use FTP to replace the logo.png which is in ol_.../images/logo folder with yours. Your logo should be the same name logo.png and as png file.
After login your joomla backend. Go in extensions - template manager and click on the template.
In template parameters - logo tab select Default Logo.
Put the height and width the same as you logo.
Click save button.