I had to modify similar folding tables to increase the height.
My method was to remove the plastic caps from the table legs and insert the needed lengths of 1/2 inch diameter metal electrical conduit to acheive the desired height.
The ends of the conduit were then inserted into 30" lengths of 2 X 4 that had 1/2 inch diameter holes drilled partway in them to accept the conduit pieces.
Drywall screws drilled through the edge of the 2 X 4 into the conduit completed the assembly.
When the tables were to be closed up the U-shaped wood/conduit assemblies were easily removed from the table legs.
Hope that helps.
