Change current directory using NERDTree
You're looking for the cd mapping. Type this inside the NERDTree sidebar, and the current directory will be changed to the current entry's. See :help NERDTree-cd.
If you're inside the NERDTree buffer you can select the folder and press C to change the NERDTree root directory. Just type :help NERDTreeMenu and go to line 252 typing 252G or 252gg to check it yourself.
Press "t" when the cursor is on the folder, it will switch to a new buffer with the nerdtree in the subfolder you have selected