Yes, it is quite possible that the defendant’s offer to you could be construed and charged as an attempt at witness tampering. Yes, if accept the bribe, and change your testimony as a result, you could also be charged.
You ask in question 1 if you can change your testimony based on new evidence. Yet, in question 2, you ask if there is any way for you to see what evidence the police/prosecution has. So, it appears that the only “new evidence” you have is what the robber who is attempting to bribe you to change your testimony is TELLING you. That is NOT new evidence, that is illegally coached testimony and, if you repeat it under oath, perjury.
You already know the answers to your questions. You know what the defendant is proposing (and you are contemplating cooperating with) is wrong and illegal. You are only hoping that someone will tell you it is ok to sooth your conscience.

