how can i return ios6 for my iphone4.

i want to return using ios 6 on my iphone 4. it is very slow for ios 7. i have to options. first solution is return to ios6, second one is start using a cheaper android device. i hope to success on first one...

At first i get a back up file of my iphone on itunes. (i use macbook) And then i open iphone in ufm mode. then i click to restore with pressing options key.

i have iPhone3,1_6.1.3_10B329_Restore.ipsw file. i selected it and i restored my iphone. while itunes was processing there is "This device isn't eligible for the requested build" error.

i stuck on this step. what is the way to return ios 6.1.3 from ios 7.0.3 ?

i need help immediately please...