if someone doesnt press charges at time of assult and a year later cahanges there mind can they do that

Although I do not practice law in the state of Texas, here are my initial impressions.  First, in most states, the decision whether or not to arrest/prosecute rests with the police/prosecuting authority, not the alleged victim.  Thus, when you state that the person has waited one year to "press charges" it sounds to me what you are saying is that the person has waited one year to report the alleged crime to the police.  What this means, from a practical perspective, is that the officers will likely take the report less seriously due to the passage of time between the alleged incident and the report.  However, this does not mean that the officers CANNOT arrest you for a crime that happened a year ago, it just makes it less likely.  If the assault was serious, and there remains strong evidence/proof, it is entirely possible that you could still be arrested.  While the passage of time may help you establish a defense (i.e. negate probable cause) if and when you are arrested, it will not provide a complete defense either unless the statute of limitations for the crime that you are charged with has passed.  Nevertheless, I recommend that you consult with and/or retain an attorney for the investigatory phase of this incident in the interest of preventing it from culminating in an actual arrest.  

Technically they can but whether or not the DA would choose to prosecute due to the intervening time period I'm no so sure.  There would have to be a very good reason as to why the alleged victim waited so long to report the incident.  Also, it would depend on just how strong the other evidence of the case is.