Can you change a mountain bike with 26 tires over to 29 tires?

You can't change the tires ONLY, as 29er tires wouldn't fit on a 26" rim.

If it's a disc brake bike, you MAY be able to fit 29er wheels in a frame/fork that originally came with 26" wheels though.

But you'll have to check how much clearance you have at the fork crown, down by the chain stays and by the seat stays. A 29er wheel will have the tire running about 2 1/2 " higher in the fork, higher between the seat stays and closer to the bottom bracket.

If you've got the space for that,(remember to check with the suspension compressed) then you may attempt the switch.

The bike will ride a bit higher, be slower off the mark, wheels will be weaker/flexier and you'll basically lose your 1-2 lowest gears.