Typically they are self righting so I would say let it move around and eventually the byssal threads will attach to a rock assuming it still has a byssal opening. Once they reach sexual maturity they lose the byssal attachment and some don't even have one to begin with.
That said, the byssal opening itself is irrelevant because the base of the shell is innately the heaviest part of the clam so they can open and close their mantle to face the sun as needed, and then use their foot to bury themselves deep enough such that it's a non issue.
If the clam is moving around, if might just be that it's in too high flow. Because Derasa tend to lose their byssal threads and or opening as they age, these are often clams that are found detached from the reef on the sea floor in slightly less well lit areas, sometimes.
If the flow is causing it to fall over, it's too much flow. They won't fall over voluntarily unless it's to move to what they think is a better place to grow.