People get very confused about these two prepositions, but there are a couple of simple tips you can use to help yourself remember how to use them.

In, as a preposition of place, is usually used to talk about the position of someone or something inside large places such as countries, continents, big cities etc.

For example:-
She grew up in New Zealand.

At, as a preposition of place, is usually used to talk about the position of someone or something inside small and unimportant places such as villages, small towns etc.

For example:-
I’lll meet you at the pub.

At is also commonly used with proper names such as buildings or organizations.

For example:-
I first met her at Harrods.
She works at the bank.

In, as a preposition of time, is usually used when talking about parts of the day.

For example:-
I’ll see you in the morning, in the evening, in the afternoon.

At, as a preposition of time, is usually used with clock time.

For example:-
My train leaves at 6.30 am.

!Note – As with most so called “rules” in English there are exceptions, so just to confuse you we always say “at” night.