Here is one way to flip a plane's vertex data or UV horizontally (X/U). Simply rem and unrem the desired lines to perform either operation.
planeobj = CreateObjectPlane(2,2)
CreateMemblockFromObjectMesh(1,planeobj,1)
NumVerts=GetMemblockInt(1,0)-1 ` number of vertices
for z = 0 to NumVerts
dx#=GetMeshMemblockVertexX(1,z)
dy#=GetMeshMemblockVertexY(1,z)
dz#=GetMeshMemblockVertexZ(1,z)
du#=GetMeshMemblockVertexU(1,z)
dv#=GetMeshMemblockVertexV(1,z)
` SetMeshMemblockVertexPosition(1,z,-dx#,dy#,dz#) ` to flip vertex on X
SetMeshMemblockVertexUV(1,z,1.0-du#,dv#) ` to flip UV on U/X
next z
SetObjectMeshFromMemblock(planeobj,1,1)
DeleteMemblock(1)
Flipping the vertex on X will likely still result in the same texture appearance, but lighting will change. So if you're trying to mirror the texture/surface details of a plane, then you may want to just flip the UV as its easier and will probably achieve the effect you are after (mirror effect in appearance).