replace
if (dbKeyState(32) == 1 ) { vx = vx + dbCos(angy); vz = vz - dbSin(angy); }
if (dbKeyState(30) == 1 ) { vx = vx - dbCos(angy); vz = vz + dbSin(angy); }
if (dbKeyState(31) == 1 ) { vx = vx - dbSin(angy); vz = vz - dbCos(angy); }
if (dbKeyState(17) == 1 ) { vx = vx + dbSin(angy); vz = vz + dbCos(angy); }
with
if (dbKeyState(32) == 1 ) { vx = vx + 5*dbCos(angy); vz = vz - 5*dbSin(angy); }
if (dbKeyState(30) == 1 ) { vx = vx - 5*dbCos(angy); vz = vz + 5*dbSin(angy); }
if (dbKeyState(31) == 1 ) { vx = vx - 5*dbSin(angy); vz = vz - 5*dbCos(angy); }
if (dbKeyState(17) == 1 ) { vx = vx + 5*dbSin(angy); vz = vz + 5*dbCos(angy); }
or in place of five put whatever else you would like the speed to be multiplied by. That will only change your movement speed though not you jump or fall speed. I haven't really looked at how gravity and jumping is handled in that sample but I noticed there is a variable for gravity so I'm assuming it should work to just change that.