How do i replace timing chain on mazda b2600 truck?

Bring the engine up to top dead center on number one cylinder. You can do this by taking out the plug and put something into the hole like a wad of paper. turn the engine by hand untill the wad pops out, then the cylinder is on the firing stroke. turn it by hand until the piston is at the top. The key on the crankshaft should be at the top of the crankshaft. Take off the timing case, remove the chain, be careful not to move the canshaft. You should be able to see the timing marks on the camshaft and the crankshaft, they should be lined up. if it is a twin cam, make sure that non of them move. Put on the new chain and re-assemble.
Good luck.