You could say: - Stay on trend. - Keep up with fashion trends.

The standard expression with the word 'fashion' would be 'keep up with the latest fashions'. (it could have a positive or negative connotation - in my context it would usually be somewhat negative) She buys quality clothes but doesn't make any effort to keep up with the latest fashions. If you want to make suggestions to someone concerning their way of dressing, you might say 'You need to dress better.' 'You should dress more stylishly.' 'I recommend that you update your wardrobe.' You'd be more likely to use the word 'fashion' if you are in the clothing industry, keeping in mind that it has a somewhat negative connotation these days.

You could say something like this: You should be fashionable. You should follow fashion. You should keep to the latest fashions. You should stay fashionable. (I'm not sure I agree with these statements, though. 🤔)

You might say …stay with current fashion trends