Heat the leather up and they will stretch and wear them around your hous
may sound weird but if you put them in the shower where it gets really steamy (but not soaking wet) it will stretch out the leather. as soon as u get out of the shower put them on while they should still be stretched out
You can take them to a boot repair place and they can stretch them for you
By "don't fit" do you mean they don't zip or that they're tight fitting? If they don't zip, you're pretty out of luck there, the best you could do in that case is take them to a cobbler and have them stretched professionally. If they're just a little tight, get the shaft pretty wet and wear them until they dry.