Give your manager a reason to agree with you. Why do you need something fresh? Are you losing your audience? If your not losing your audience yet, are there reasons why you will soon? Is the site starting to look dated, are you getting negative customer feedback, is there a competitor out there that's snatching your audience, and if there isn't currently, is there space for them to do so?
Most likely your manager is currently thinking that the established design works, and that deviating from it is risking failure. His/her logic is sound to a certain extent, but stagnation can cause serious problems very suddenly and unexpectedly.
One example being Paypal. It was an ugly website styled straight from the corporate heart of the 90s, which a lot of trendier websites were forced to integrate with for technical reasons. Suddenly Stripe comes along, which does exactly the same thing, but looks good while it does it, and everyone's using Stripe instead of Paypal. Cue PayPal not only having to redesign to look sexy (which it should have done before it lost all its customers), but also spending millions of dollars on ad campaigns focused at getting their lost customers back. That's a lot of lost cash due to stagnant management. You can see similar examples with Hotmail and Gmail and Internet Explorer and Firefox/Chrome, although those two examples are more functional than aesthetic.
So the question is, is there a danger of that happening with your product? Is it starting to look dull? Is there a possibility of a better designed product snatching your business? If so, that's what you need to be chatting to your manager about. You saying it needs to look "fresher" doesn't mean much in terms of results for your manager, to him or her it sounds like risk and work. Show them there's a risk in stagnation too. And a reward for innovation.
This is still something of a stab in the dark, because I don't know who you work for or what their competition landscape is, but hopefully it helps. You can always de-risk it for your manager by asking for budget to take your favourite designer to the side for an experimental design, and then offer that to users as a "beta" site they can test. If you can produce the new product cheaper than the established one that will be an incentive for management change.