Is a contract valid with wrong working times and wrong payrate?

I assume that it is you who want to consider it invalid, not your employer.

In no way can your contract be considered invalid by your employer if he changes your hours or pay. For example, if you have a 1-year contract and halfway through your employer raises your pay and then a month later he says the contract is invalid, that would be incorrect.

It is easy to break a contract if both parties agree. If you all agree the times and pay rate are wrong, and you all agree that the contract is obsolete, then you all can write a new contract.

If the hours and pay were changed with your agreement awhile ago, then it is too late to complain and the rest of the contract is probably still valid. You could consider forcing the issue by working those contract hours and at that pay. By contract, you are OK to do this! If your employer doesn't like it, then you can suggest the contract be changed or renegotiated.

If the hours and pay are being changed today, then it is breaking the contract and you have 4 choices: 1) You can insist that contract stay as it is, 2) you can ask for the contract to be updated with the changes, 3) you can consider this an opportunity to re-negotiate a completely new contract or 4) you can use it as an excuse to leave.