iphone 3gs updated to ios 6.1.6 by jailbreaking but while doing so i have unchecked install cydia

Hi ,

I have updated my iphone 3gs to ios 6.1.6 but by mistake I have unchecked the install cydia. Is there any possibility to install cydia explicitly orelse I have to downgrade ios.

Actual problem :

I have successfully updated ios to 6.1.6 but I am unable to work with my sim as there is an error "insert sim " so to rectify that I have to install cydia.

Is there any solution for my problem , Please provide.