There is more than one dietary factor that affects your blood pressure, but if you were to make only one change, you should reduce your sugar intake.
Research lasting 8 weeks or more showed the greatest changes (Te Morenga LA, et al. 2014).
Systolic blood pressure was in average reduced with 6.9 mm Hg and diastolic with 5.6 mm Hg.
This was independent of weight loss which also has shown to reduce the blood pressure.
If you made more changes and switched to a plant based diet with a low glycemic index, you can expect to see really big results.
The longer you eat that way, the more benefits you will see.
If you were taking blood pressure lowering medications, you would most likely have to reduce what you take, or maybe even stop taking them all together.
You should tell your doctor what changes you are making and be sure to have check-ups.