To change a string you must login to the administration area of the website and navigate to Extensions > Languages > Overrides.
From this view, you can select if the language string you want to change is in the front end or the back end of your website.
From the Filter menu, select what you want and then click on "New". The New override view has a list of options that you must fill in with the desired values.
From this field you can select the search factor. You can select between "Value" and "Constant". Probably you don't know the Constant of the field so leave it as Value.
Type your desired text here and press search, In our example we typed Log in because we want to change the module log in button to something else.
As you see in the above image typing Log in and press search, opens up a list of results that contain the string "Log in". You must click on the result you want to change. In our example is the 6th result "JLOGIN". So click on it and on the left part of the page the fields 3 and 4 (image 2) will get filled with the JLOGIN values. So you simply change the result on the "text" field and then click save and close.
Thats it! The Log In field has your new value.