Without code:

1) Go to StarterPlayer in the explorer. (view tab->explorer)

2) In the properties, (view tab->properties) look for the WalkSpeed property. It should have a default value of 16.

3) You can edit this number to be any number you want.

With code:

PlayedAdded and CharacterAdded are events.

PlayedAdded runs when the player joins the game CharacterAdded runs when the player's character loads into game

inside of character added, we change the players walkspeed. this means whenever the player's character loads, their walkspeed will be changed to 'newWalkspeed'

local newWalkspeed = 10 

game.Players.PlayedAdded:Connect(function(player)
   player.CharacterAdded:Connect(function(character)
      character:WaitForChild("Humanoid").WalkSpeed = newWalkspeed
   end)
end)