If you just want to fit langmuir to the data, you can use CAVS - adsorption evaluation. There is a whole module for isotherms in this software (Langmuir included).
check the software here: http://bit.ly/CAVS_tutorials
Your use of the word "fix" is confusing but if you mean how to find the Langmuir coefficient, you may use the "analysis" function on the Origin software. Alternatively, the Microsoft Excel may be used as well. The step-by-step processes for these are usually not described in papers but you can see some explanations on youtube