Small Business Contract Creation

We work for a bigger company as subcontracts and the bigger company keeps changing the contract with speaking to us and we already have a contract that they have changed with consent form us on any occasion's without consenting which is in our contract with them so we are seeking guidance with this problem as now they have changed our pay days making from 7 days to 14 days without our premission both have to agree which we didn't, please help us flight this injustice for the little people. We are sick of being BULLIED by them.
Thank you
Kind Regards C&V TARRANT TRANSPORT
Phone Vicki Tarrant on 044 7051911