In regular ARM mod, you could just the pc to overwrite any instruction. In THUMB it says it's not allowed (cause the pc is r15)
(the pc stands for program counter, it basically tells where the next instruction to be executed is)
In ARM everything is 32 bit though, including the instructions, so it'd take up more room, but it's definitely more powerful.
Like, let's say in ARM you wanted to overwrite the very next instruction. Let's say r5 has the instruction you wanna write. To overwrite the very next instruction you'd put....
str r5, [pc, #-0x4]

to overwrite the instruction after that instead
str r5, [pc]
etc.