Set as default / Set to default?

Solution 1:

It depends on what you mean.

If you want to set the current value as the default value, for future use or something else, set as default would be the correct text.

If you mean, to set the current value to the default value (i.e. changing it to some other, previously specified or natural default value) then set to default would be what you require.

Solution 2:

Here's the math way.

Set as default = What you see on the screen RIGHT NOW, will become the new setting.

Set to default = What the device was set to initially by the manufacturer or programmer (or last teacher, lol) will be restored.