I don't know the specifics of your computer but somewhere along the line you'll have to tell it the circumference of your tyre.

In very simplistic terms the diameter of a 700 wheel is very roughly 10% more than a 26" MTB wheel, so the circumference is also going to be around 10% bigger. If the default setting for this computer is a 26" wheel then everything it reports will be 10% too low. If the default setting is for a 700x32 and you're running on 700x35 the difference will be much smaller. But even then I'd be minded to ask what the point of measuring your speed is if you're not going to measure it with at least a degree of accuracy.

In terms of repositioning it you can cut zip ties with wire cutters without damaging the bike at all. Just be careful where you cut. Then you can use fresh zip ties to secure it wherever it needs to be. It's really not difficult.