If you have a membership and wish to increase the price, you can do so inside the product builder by editing the price.
The new price will be reflected at check-out and will only apply to members signing up AFTER the price change.
Members who paid the lower rate, will not be affected by this price change. They have an existing subscription with you at the lower rate and that will not change unless you or they stop the subscription. If they do stop the subscription and resubscribe, they will be charged the new, higher rate.