If you leave your Skybox connected to your TV (without a subscription) you'll still be able to receive all of the channels listed here via your satellite dish:
https://en.wikipedia.org/wiki/List_of_free-to-air_channels_at_28%C2%B0E
Since almost everything which is available on Freeview is in that list, that's probably all you need to do (or, rather, not do).
However if you'd rather switch to Freeview, then you simply need to plug a terrestrial aerial into the relevant socket on the back of your TV set. (It's irrelevant as to whether you disconnect your Sky box or not. You can use both means of receiving TV signals if you want to).
Then you simply need to change the 'Source' setting on your TV from 'HDMI1' (or similar) to 'DTV' (or similar) by using the TV's remote control.
The 'Source' button is often marked by a rounded rectangle, with an arrow pointing into it. 'DTV' might also be shown as 'Aerial', 'Ae', 'Ant', 'RF' or something else.
Note that if your TV set hasn't previously been tuned to receive terrestrial signals you'll also need to go into the Settings menu to scan for channels. (That's best left until tomorrow though because lots of channels are moving about then).