Your value relative to your loan is only an issue if you want to sell or borrow against your home. If you believe you are only $15k from being above water, then you are in pretty good shape compared to many others.
The answer to your question depends on your long term plans. Upgrades made today will be dated and worth less in 10 years. If you hope to sell soon, then it depends on your market, the upgrades and degree of need.
A fully functional kitchen may cost more to upgrade then it will repay. A badly dated or dysfunctional kitchen may be a huge hindrance to selling.
If you plan to stay a while, do the necessary maintenance and pay down the mortgage. If your rate is high there are options to refinance now that you should consider, even in your current status. Just consider whether you want to start with a new 30 year loan or to do a shorter term like 25, 20 or 15. You'll make more progress faster with a shorter term.... more
If you are interested in buying a house you should have an agent that you're working with. One of the things that your agent will be able to do is see if a previous listing might still be available. I don't think the zestimate means any more that what it says, an estimate of value.... more