Never tried this myself but I read somewhere that if you wear big fuzzy socks that are wet inside your boots then blow dry them they will stretch .
Walk around in them in the house a lot! Expect blisters but it will break them in without getting all dirty!
One pair of tall dressage boots were particularly naughty and uncomfortable. I resorted to getting into a warm bath with them and wearing around the house until dry. The leather around the ankle would fold and 'bite' into my ankles - so painful that I also tried the rubbing alcohol trick, but would shove an empty magnum bottle down each ankle. For months afterwards, I couldn't understand why the ankles still hurt so much, despite thick socks, until I realized that in shoving the bottles down the ankles, I had bent the seams on the inside of the boot in such a way that it rubbed horribly. I covered the seams with fabric tape (for first aid), and all is well now.
I run up and down the stairs!! 😂 It works tho
Use a 50/50 mixture of rubbing alcohol and water, spray the inside of the boot anywhere it is tight (ankles especially) and ride in them and let them dry on your leg. Give it a good spray, so it is more than damp, but not soaking.