Go to TANDY (Leather Store not the Radio Shop), and buy some leather cleaner stripper and also buy some BROWN LEATHER DYE (the commercial grade solvent based dye not the water base stuff).
Clean boots with leather cleaner stripper and redye with the leather dye. (Read instructions first or ask for advice from Tandy Staff).
Test on a out of sight area first i.e. the boot tongue as I have seen a pair of brown boots go purple!!! (because the individual did not read the instructions).
Or a cheaper option go to Morrisons (other supermarkets sell their own band) and buy the cheap £1 scuff cover liquid polish in brown (lots of).
Clean boots in white spirit or similar and apply scuff cover. (Again test in a out of sight area as I have forgotten which method turned the boots purple!!!!).