I saw something on Facebook the other day - so it MUST be true
Take a Ziploc bag, partially fill it with water. Seal it, insert into boot, insert boot into freezer. As the water turns to ice and swells, so the leather is stretched.
I currently have a pair of Dubarrys residing in my freezer because I thought "well, what the hell, they cripple me now, so if I break them it can't be worse"
Of course, you could always search high and low for a cobbler (do they have shops any more?) and ask his professional advice
If you can bear it, put on a sodden pair of socks and then the boots.
Works well for stretching the foot section.
I got a pair yrs ago to fit like gloves after taking a tumble out hunting into a deep water fence, filled my boots (literally!) But after careful drying hours later, the boots were a perfect fit instead of pinching my toes like they did previously !
It was horrible at the time and i got a blister! But totally worth ut
Have used similar since then, but with warm soaked sock and then stomping about house and garden for a good hour or so, till the leather on the outside feels damp.