Changing to a smaller gear ratio I have a Kona Rove ST, with a 40 teeth gear wheel and a 11-42 cassette. It's all SRAM. Here is the technical description.

It's ok for now, but I'm often using the smallest ratio (40x42) so I don't have any margin. Hence, I'd like to have a smaller gear ratio. To make it more complicated, I like my biggest ratio just fine, but I guess I'll have to compromise and lessen it.

But lowering my smaller ratio seems quite complicated and/or so expensive. So I thought I could just change the gear wheel for a smaller one. It seems straightforward to replace it with a 38 teeth one, but I'm not so sure for a 36 teeth one. 

So my questions are: is it worth it to replace my gear wheel with a 38 or 36 one, and if so is it easy to do so? (especially for the 36 teeth)
Or is there any other solution that would work?
Since it's a 1x setup you don't have to worry about the front derailleur, so it's definitely possible to swap in a smaller chainring.  That will affect your highest gear too though, so you should bear that in mind.

I don't see any reason for there to be a problem switching to a 36t chainring, noting that 36x42 is only 10% less than 40x42.  Maybe that's enough of an improvement; it's up to you to judge how much easier you need it.  It will reduce your max speed - for a given cadence - by about 10% as well in your top gear.

The SRAM X-Sync chainring has a 5-bolt, 110mm bolt-centre diameter (BCD) mounting, (sample link only) so you need to find a compatible chainring.  Again, not to make a recommendation, but a quick search found a 36t Wolf Tooth 1x chainring with a matching bolt configuration.

It is possible to find a cassette with a bigger range (though not a SRAM 11-speed) but that would require you to replace your rear derailleur as well, since the SRAM Rival 1 derailleur your bike comes with has a max of 42.  This is almost definitely not worth it.