-Providing that the image shown is the tripod you own..., the top leg lock levers on each leg is designed to lock the spreader angle of each leg.
-If this is not holding the leg at the correct angle...., either you are putting more weight on it than it was designed to hold..., or the 'cam' on the various locks has worn.
-A 'stop gap' fix for this problem is to insert a very thin wedge between the lever & the leg tube..., thus providing a greater amount of pressure on the tube, (this would need to be done every time the tripod is set up).
-Replacement parts are not available for this tripod & they were not designed to be user serviceable ...., sorry!!!
Peter