They make a boot stretch spray which
Is basically just pure rubbing alcohol. Spray it on your boots and flex your calves and walk around until you can button your boots. Then you can just keep doing this so it's more comfortable.
Yeah puts lots of warm water on the inside and outside with a sponge to really get the leather soaked. Then put them on and zip them up as far as possible, wear them for a few mins then try to zip them up further. Wearing them when wet can be uncomfortable, and it takes time but it makes a huuge difference