how can change internet connection

in my office we are using 4 ISP lines, whenever ,one ISP GOING DOWN we are using other line, but i feel difficulty to change the gateway address of all systems., all isp is connected to our switch , so always i should change i should gateway address all systems from one isp gate way in to other isp gateway , we have 45 systems in our office

so is there any software , i can install and change one isp to other isp at a time , or any other possibily

pls reply to me