There are several nutritional ways to attempt to manage blood pressure. First, if you are severely overweight and do not exercise, it is doubtful that nutrition alone will alleviate the condition. You are advised to address those issues as well. That being said, there is much that you can do nutritionally to start down the proper path towards hypertension management.
A subset of hypertension sufferers are sodium sensitive. This means that excess sodium causes their bodies to retain water and increase blood volume, thus increasing hypertension. Sometimes it is an easily treated potassium and/or magnesium deficiency, and not excess sodium. Sometimes it is excess sodium but more potassium, calcium, and magnesium would balance out the electrolyte profile and cause the body to relax the blood vessels and not retain water since now the excess sodium could be flushed out easier. The body is a complex device and in some cases the solution is a simple one. You have to experiment with some of these things and may be lucky enough to happen upon the answer for your particular root cause.
Another dietary aid which may help your hypertension is a supplement or tea prepared from Hawthorn berries. Hawthorne berries are thought to relax the blood vessel walls, working similarly to some prescription hypertension medicines. Of course the effect is not as great and takes some built up use to manifest, if at all. Some people claim it works for them, while others say no.
Magnesium supplementation is something that anyone with high hypertension should at least try. It has been shown that those suffering from hypertension are usually low in magnesium. Magnesium relaxes the blood vessel walls, dilating them for easier blood flow and reduced pressure. Magnesium citrate is the preferred form for optimum absorption, and 400mg per day is the upper limit. If you go too far beyond this you might end up with intestinal upset and loose stool, so be careful. If you wish to experiment beyond the 400mg threshold, do it in divided doses, morning and night, and take it with a meal.