If $a=?iso-8859-1?Q?=2Cp\in\mathbb{N}$_with_$p$_prime=2C_have_to_show_that_if_?==?iso-8859-1?Q?$a=B2\equiv1\pmod_p_$=2C_then_$a\equiv1\pmod_p$_or_$a\equi?=v p-1\pmod p$

If $a,p\in\mathbb{N}$ with $p$ prime, have to show that if $a²\equiv1\pmod
p $, then $a\equiv1\pmod p$ or $a\equiv p-1\pmod p$

If $a,p\in\mathbb{N}$ with $p$ prime, have to show that if $a²\equiv1\pmod
p $, then $a\equiv1\pmod p$ or $a\equiv p-1\pmod p$
I'm studying congruence, and I have no idea where to start this
demonstration, if anyone can do it, preferably detailed, or go giving
ways, and I doing, I will thank enough.