How do I know when to change exercises in weight lifting?

Anonymous
I've heard that if you do the same exercise for 2-4 weeks or more, your body adapts to the stress and you need to change something. Assuming I want to keep the weight as it is, as well as the sets and reps, the only thing left to change is the exercise itself. So when do I know when to do it? Should I keep going until I feel like the exercise is too easy (ie I can do all the assigned reps easily without being forced to give up on the last one)?

Also, imagine that I decide to change exercise 1 to exercise 2 this week. Next week can I go back to exercise 1 and keep changing them like that?
Thanks for answering in advance.
How do I know when to change exercises in weight lifting?
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