I have bmw 525i '06, the tires is 35 series with 19 inches wheel, can I used 40 series tires X 19 inches

Yes u can, they will fit. But why would u want a 40 sidewall? To keep the diamater of the wheel the same as the original, u will need to use a narrower tyre, so have les contact with the road & less grip. This may invalidate ur insurance. Why would u want to do that?

The 19 inch wheels BMW provide for the 5 series have the following tyres:

Front - 245 / 35 / 19 (25.8 inch diameter)
Rear - 275 / 30 / 19 (25.5 inch diameter)

When u buy a different size tyre, u need to try to keep as close to the original size so that the speed displayed in the car is fairly accurate.

If u choose 40 sidewalls that keep the diamater close to what BMW would provide, you'd need:

Front - 215 / 40 / 19 (25.8 inch diameter)
Rear - 225 / 40 / 19 (26.1 inch diameter)

If u keep the same width & go for a 40 sidewall:-

Front - 225 / 40 / 19 (25.1 inch diameter)
Rear - 275 / 40 / 19 (27.7 inch diameter)

These will fit BUT this will raise ur car so far off the ground that it will look like it is on stilts. Not a good look.

PLEASE CAN YOU EXPLAIN WHY YOU WANT 40 SIDEWALL TYRES - IT MAKES NO SENSE.....

NO look into what is compatable with the car radius,diameter,wheel travel you kids dont look at all these little thing and then you wonder why dont it work. geee looks ok l get it but at least make it ride and look good