I accidentally set shapekey value to 1 while editing mesh, but I actually wanted to it to be 0.5 How can I change shapekey value and keep it's mesh shape? I want to slide to 0.5 value to get my mesh shape, not to 1 to get it.
Under the Value slider adjust the Range's Max to 2 and increase the Value to 2. Every other Shape Key value must be 0.
Then click on the little triangle under the minus button and select New Shape From Mix. This will create a new shapekey with what you wanted. Delete the original and reset the settings.