How can you increase your blood pressure?

Salt,see below. The fastest way is salt, caffeine, decongestants. Aging and gaining weight work but they are slower. But there is no need to do any of this unless you are symptomatic from hypotension with dizziness or fainting.
Fluids & salt. Blood pressures 90/60 or less may be normal for many people unless they develop it suddenly or recently. If the blood pressure was high or normal140/89 for no diabetics and 130/ 85 for diabetics and now is low could be secondary to dehydration, bleeding ( hidden bleed) or medications. Try fluid intake, increase sodium intake, elevate legs at the level of the heart for 30-45 min several times a day.