I have some tall grass in my back yard and I would love to cut it but I am worried I could kill it by cutting like more than 80-90% (Hopefully not exaggerating) will kill the plant. Should I try to slowly cut it down to size or would it be fine if I gave it enough water. I would like to not have a brown patch in the back yard, even though it will not be seen to easy.
The lower parts of the grass probably aren't green right now because they're not getting much light or good air circulation.
It's usually not recommended to cut more than 1/3rd of the grass at a time. So you'd cut it down to 24", wait a few days then cut it down to 16", wait a few more days and cut it down to about 11" and so on. Well take about 6 or 7 cuttings depending how fast it grows in between cuttings.
With grass that high though it still may have other issues.
If it were me I'd just cut it down all in one shot or maybe 2 because it looks bad. Maybe bring it down too about 1ft with a string trimmer then about a week later go through the area with a take to clean up rocks and other debris and hit it with the mower on the highest setting then work down to your desired height.
Try to avoid mowing when the temp is greater than 90f. Start maintaining the lawn after you cut it. Give it some fertilizer and mow it regularly. Outs not going to l look great right after you cut it but it will recover. In the fall overseed it to help fill in any week areas which you'll undoubtedly have.
There was a similar question recently and you might find my answer to that one helpful as well. https://gardening.stackexchange.com/a/25308/2448
Turf care position: Oh Noes!!! Also, 2" is too short, unless you are going for a golf-green, which is a whole other level of fuss (and even shorter.) 3" is about as short as you should cut normal lawn grass. Among other things, cutting the top too short kills off the roots which makes it less able to get water for itself. Taller grass equals deeper roots.
Practical "whacking the overgrown stuff in the orchard": it will probably live. I can't say I care much if that stuff turns a bit brown for a while, so I suppose it might. But you might want to put your mower as high as it goes, then give it a week, then cut it back to 3". But hayfeilds put up with "several feet" to "a few inches" multiple times a year without expiring....