Can I convert hybrid to mountain bike. Tyres and tubes?
There's a bunch of weird standards for naming the size of tires and rims, and there's an actual ISO standard.

The best thing to do is ignore the big letter printed names that have inconsistent conventions, and look for the ISO standard stuff, like "42-622".

The ISO standard size of your tires and rims is 622.  (18mm wide rim and 42mm wide tire)

ISO 622 is the same as "700c" (common road size) and 29er (new larger mountain tire).

In other words: yes, a 29" tire should work fine, as long as it's not too wide. 

Try to stick with something close to the same width and they'll work fine with your existing rims. Mountain (off-road) tires with their knobs can require more clearance at the same size.

As long as you keep to about the same size tire, your existing tubes should be fine.