Whether the boots are leather or synthetic the treatment is the same
Treat them with some sort of leather grease and then knead, fold, or 'work' the offending areas with your hands & fingers to soften them
Walk through streams as much as possible, getting them soaked, and walking in them wet for a day. This will stretch & form the boot to your feet
It's summer, so there won't be a problem with cold feet after the initial soaking
If the boots rub around the top of your ankles, protect them by smearing a sanitary towel (I kid you not!) with antiseptic cream or Vaseline and taping it in place. This tip was given to me by a chiropodist