Angles can be measured in units of degrees or radians. A complete revolution is defined as $360^\circ$ which is equal to $2\pi$ radians
\[360^\circ = 2\pi \text{ radians.}\]
From this, we can derive that
\[1^{\circ} = \frac{\pi}{180} \text{ radians.}\] \[1 \text{ radian} = \frac{180}{\pi}^{\circ}\]
Sometimes a superscript c is used to denote radians instead of degrees, though it is conventional to assume that radians are used unless otherwise specified.
Convert $83^{\circ}$ to radians.
Recall that $1^{\circ} = \dfrac{\pi}{180} \text{ radians}$. So multiply $\dfrac{\pi}{180}$ by $83$:
\begin{align} 83^{\circ} &= 83 \times \frac{\pi}{180}\\ &\approx 1.449 \text{ radians (to 3 d.p.)} \end{align}
Convert $3 \text{ radians}$ into degrees.
Using the definition, if $1 \text{ radian} = \dfrac{180}{\pi} \text{ degrees}$, then multiply by $3$ to find the angle in degrees.
\[3 \times \frac{180}{\pi} \approx 172^{\circ} \text{ (to 3 sig.fig.)}\]
Prof. Robin Johnson shows how to convert $37^{\circ}$ to radians, and $1$ radian to degrees.
This workbook produced by HELM is a good revision aid, containing key points for revision and many worked examples.