The first option is to develop an API layer over your old system, that is supported by a full integration test suite, and then either build a new frontend using that API, or port the old frontend over to that new API. As this effort progresses, your integration test suite and well defined API allow you to replace/rewrite portions of your backend as well.

The second option is to look at your most valuable customers, discern what features they use and rely on, and design a fully separate clean slate implementation of only those features in a new "V2" product.

In either case, what you're describing is a very tough situation, and any solution would be very risky. Management usually is not aware of tech debt, they might not know they've accrued a million dollars or more of engineering debt that they will have to pay off to solve problems like being able to quickly respond to new market developments, retaining high quality engineers or ballooning customer dissatisfaction.