to touch or fiddle with something (with one's hands); to fidget with.
スマホをいじる時間が長すぎる。
You spend too much time fiddling with your smartphone.
彼は髪の毛をいじって落ち着かない様子だ。
He nervously fiddled with his hair.
生徒が机の角をいじっていた。
The student was fidgeting with the corner of the desk.
to tinker with or make small changes to something (settings, code, machinery).
コードを少しいじってバグを直した。
I tweaked the code a bit and fixed the bug.
車のエンジンを素人がいじるのは危険だ。
It's dangerous for an amateur to tinker with a car engine.
設定をいじってみたが、問題は解決しなかった。
I tried changing the settings, but it didn't solve the problem.
to dabble in something or do it as a hobby; to mess around with (nonprofessionally).
週末は写真をいじるのが趣味だ。
I spend my weekends editing/playing with photos as a hobby.
彼は庭をいじるのが好きだ。
He likes tinkering with his garden.
to tease or make fun of someone (colloquial); to poke fun at.
みんなで冗談を言い合って友達をいじった。
We joked around and teased our friend.
上司にいじられて恥ずかしかった。
I was embarrassed when my boss teased me.