In fact, it could hardly be easier: assuming your new mouse and your old mouse both plug into USB sockets, you simply unplug the old one and plug in the new one. There’s no need to have the PC switched off when you do this, but equally, if it’s currently switched off, there’s no need to switch it on first either.
If either the old or new mouse plugs into a different type of socket (such as a circular ‘PS/2’ socket) , you should do the switchover with the PC switched off.
The nice thing about mice is that their basic job is so simple that Windows can automatically install the software required to make them work. As soon as you plug in the new one (and start your PC, if necessary), it should be usable – at worst, there may be a pause of a few seconds while Windows sets it up.
One little wrinkle is that your mouse may have additional features beyond those included on the most basic mice: it may have extra buttons which can be configured to do particular things when clicked, for instance. In that case, you probably received a CD of software with the mouse, and you should install that software before plugging in the new mouse.
So, to recap, the routine is this: install the software accompanying the new mouse if required; shut down your PC if either mouse is a non-USB mouse; unplug the old mouse; plug in the new mouse; switch on your PC if it’s switched off. If your old mouse had its own software to enable its extra features, you might like to uninstall that software now, just for tidiness, but don’t worry too much about it.
Incidentally, everything I’ve said about the mouse applies to the keyboard too. If you want to replace your keyboard, just mentally swap the word ‘mouse’ for ‘keyboard’ above and approach it in exactly the same way.
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