Fetching details from the dictionary...

to finger; to touch; to play with; to fiddle with; to toy with
to make changes to; to tinker with; to tamper with
to dabble in; to do as a hobby; to play around with
to tease; to make fun of
to grope; to feel around (in one's pocket, bag, etc.)
彼は緊張するとペンをいじる癖がある。
He has a habit of fiddling with a pen when nervous.
機械をいじっているうちに壊れてしまった。
The machine broke while I was fiddling with it.
設定をいじったら動かなくなってしまう。
It stopped working after I messed with the settings.