Time reversal

Continuous

Time reversal refers to the reflection of $x(t)$ about the vertical axis, and is done by:

$$\varphi (t)=x(-t)$$

Basically, we just replace $t$ with $-t$.

Another example:

$$\begin{array}{rl}\text{Original}& =x(3t-5)\\ \text{Reversed}& \to x(3(-t)-5)\\ & =x(-3t-5)\end{array}$$

Discrete

This is basically the same:

$$x_r[n]=x[-n]$$