In medic.cfg:
alias "main1"             "equip 5 0 0; bind F6 main2" //Equip Needlegun
alias "main2"             "equip 5 0 1; bind F6 main1" //Equip Blutsauger
alias "alt1"              "equip 5 1 0; bind F7 alt2"  //Equip Medigun
alias "alt2"              "equip 5 1 2; bind F7 alt1"  //Equip Kritzkrieg
alias "mel1"              "equip 5 2 0; bind F8 mel2"  //Equip Bonesaw
alias "mel2"              "equip 5 2 3; bind F8 mel1"  //Equip Ubersaw


In pyro.cfg:
alias "main1"             "equip 7 0 0; bind F6 main2" //Equip Flame Thrower
alias "main2"             "equip 7 0 5; bind F6 main1" //Equip BackBurner
alias "alt1"              "equip 7 1 0; bind F7 alt2"  //Equip Shotgun
alias "alt2"              "equip 7 1 4; bind F7 alt1"  //Equip Flare Gun
alias "mel1"              "equip 7 2 0; bind F8 mel2"  //Equip Fire Axe
alias "mel2"              "equip 7 2 6; bind F8 mel1"  //Equip Axting


In heavyweapons.cfg:
alias "main1"             "equip 6 0 0; bind F6 main2" //Equip Sasha
alias "main2"             "equip 6 0 8; bind F6 main1" //Equip Natascha
alias "alt1"              "equip 6 1 0; bind F7 alt2"  //Equip Shotgun
alias "alt2"              "equip 6 1 7; bind F7 alt1"  //Equip Sandvich
alias "mel1"              "equip 6 2 0; bind F8 mel2"  //Equip Standard gloves
alias "mel2"              "equip 6 2 9; bind F8 mel1"  //Equip K.G.B


And finally in autoexec.cfg (note: if you don't have this file, just open notepad put this in and then save as "autoexec.cfg" in "\tf\cfg"):
bind F6 "main2"
bind F7 "alt2"
bind F8 "mel2"