Q: How do you clear a specific bit in a register without changing the other bits in ONE instruction? For example, let's say register 4 (R4) has a value 0001110010101111. How could you change bit 5 (0001110010 >1< 01111) to 0 (even if it was already 0) without moving or changing the other bits in a single hex instruction?
So 0001110010101111 -> 0001110010001111

A: You'll want to AND it. Since the immediate value for AND is 5 bits and it uses sign extension, you can only clear a bit if it's one of the four least significant bits. Otherwise, you will need to perform another instruction to load the mask into a register. I'll do an example of both.
In the case of the 5th bit, the number that will mask the bit is 0b1111111111011111. In decimal, this is #65503 or #-33. Since this is too big to fit in an immediate instruction, you won't be able to do it in a single instruction. You will need to declare it in the data segment of your program and load the mask into a register. Then, you can AND it with R4.
; assuming R4 = 0001110010101111
LD R5, MASK_5  ; load the mask into R5
AND R4, R4, R5 ; set R4 = R4 AND R5
               ; R4 will now have the bit cleared

; data segment
MASK_5 .FILL #65503

In the case of the 3rd bit, the number that will mask the bit is 0b1111111111110111. In decimal, this is #65527 or #-9. This will fit in the immediate value of AND, so you can perform it in a single instruction:
; assuming R4 = 0001110010101111
AND R4, R4, #-9 ; set R4 = R4 AND #-9
                ; R4 will now have the bit cleared

