How can I stretch the cane on a chair back?

Is it sagging? The only way to tighten it up again is to remove the spline (the bamboo piece that holds the cane in the chair), pull the cane out, soften it by wetting it and restretching it. You would then have to push it back into the groove and put new spline in to hold it. You can buy supplies for doing this online. It might be easier to just replace the cane rather than reusing the existing cane.

Try spraying the cane well with hot water and let dry. It may help a lot. Be sue to have an old cloth to catch the water running off the chair and wipe the chair area several times as it dries so the rest of the chair and frame isn't damaged.

On the chair?? if on the back you'll need to lie chair down to place towel on it,place a wet hot towel (the same size as the piece) over the piece you are trying to stretch keep it damp and hot til soft/pliable(long time) if you can do both sides it'll be much better ; why does it need stretched? are you trying to remove it & put a new piece in, if it's a new replacement piece you'll need to soak it in tub of hot water for 2 hrs or more before you can use it; any grasses,reeds,bark strips,vines need to be soaked so they are pliable for weaving baskets,mats,caning etc.. here's good site http://www.basketmakerscatalog.com/mfiles/canewebbing.php