How to increase my garden bed’s soil PH level

At zone 7b. We built a new garden bed this year (4x6x2 f). Filled it with a mix of soil and sand (2:1) from a farm our friend ordered from. Then added a layer of Organic Bumper Crop soil builder as its top soil. After one month of growing, I noticed my plants (tomatoes, spinach, bokchoy, beets, carrots and strawberry) are not growing much regardless of direct seed or transplanting. I tested the soil using those electric meter and noticed its PH level is below 6 in most places and the deeper it goes the lower PH level goes. Some were even at 4.

I will test tomorrow once the home kit has arrived. If the PH level is really this low, what can I do? Do I need to take out all the plants and then amend the soil to fix the level? Can I do it while keep these plants in? I regarded trusting unknown source for soil. Should have tested them first.