I have 4 cabinet door having same issue. They are not working as powerful anymore and I need to keep the door to lift up instead of these bars. While they seem very basic devices, is there a way to fix these liftup support bars or do I have to throw them to wastebasket ?
The pictures are not enough to understand how they are built. I recommend you to analyze them as verify if they have anything that looks like "adjustable something". Maybe there is a ring that you can rotate, maybe some screw, or any other detail.
Another idea would be to find the brand and model, and then look on the internet for instructions how to adjust them.
However, be prepared to the outcome that they are cheap bastards (they usually are) and that there is nothing you can do about it. I mean, besides buying and installing new ones.
Some easy hack
Move the lower end and / or the upper towards the hinge. In this way, you will have the effect that you desire. Looking at the detail of the piston / bar, all you need to do is to unscrew a little the screws, move the end, screw again.
If the movement is no longer possible because the screw is at the limit, just apply the screw to a new position closer to the hinge and then adjust.
This operation will require some trial and error, but should not be difficult at all.