How do I clear previous values while using strcat() to add strings?
strcat always concatenates to the end of the string. Instead, for your first strcat you should really be using strcpy, which copies one string over the top of the other - that is, it places it at the start of the string, effectively starting a new string.

However, unless you really need the data in a string for some specific reason, you don't need to build a string up - just print each part to serial separately with its own Serial.print function.