Can I put a lower geared cassette on my bike?
Changing to 10 speed gets real expensive real quick, because you need new chain, cassette, shifter at a minimum.

32 tooth is 4 more than 28, so its a ~14.3% increase.  That's quite a lot - you will need extra chain links, and your rear derailleur will need adjusting and may not work with the extra teeth. It's hard to be sure without trying it.

Your other possible change is to shrink the smallest of your front chainrings.  This may mean slower changes between rings, or the FD may not have enough capacity.  You'll also need a medium or long cage rear derailleur to help take up the difference in chain length.

Final option is to grin and bear it.   Climbs up hills aren't fun.  They're supposed to be a workout.