Sometimes you can use either, but not always.

In a group,

'each' refers to individual things in turn.

'Every' refers to all the things together.

If I have time,

I say 'bye' to each of my colleagues.

If I don't,

I say 'bye' to every colleague at the same time.

If we have two things, we have to use 'each'.

If we have three or more, we can use 'each' or 'every'.

I have a shoe on each foot.

Lions have sharp claws on each or every paw.

'Each' can be used as a pronoun, while 'every' can't.

Look at those cakes! I want a slice of each!

'Every' object is together. 'Each' object is on its own.

'Each' is for two or more. 'Every' is for three or more.

'Each' can be a pronoun and 'every' can't.