Can I replace a Shimano FC-MT500 26/36 crankset with a Shimano Ultegra FC-R8000 36/46 set? On my Kross Level 10.0 MTB (2x10) used mostly on dirt roads and pavement I find I never use 1-10 and seldom go below 15th but I need more top end speed. Wondering if the Ultegra would be a straight swap out?

Thanks!
Unfortunately no, the Ultegra crankset is not compatible, nor is any other road Shimano crank designed for a Hollowtech II bottom bracket (assuming this is your bike).

The issue is that there are MTB and Road versions of Hollowtech BBs. MTB ones fit in 68 or 73mm shells, using 2 2.5mm spacers on 68mm ones. Road BBs only fit in 68mm shells. MTB BBs are therefore 5mm wider than road ones. Road crank axles are 5mm shorter than MTB cranks, and are only long enough for road BBs.

If you have a 68mm BB shell you could put a road BB in it and fit a road crank but the chainline would be way off.

You could look for larger aftermarket chainrings in the Shimano asymmetric 4-bolt pattern, but you have to deal with chainring clearance issues Andrew Henle described in his answer.