Both "ARM" written in all caps and "Arm" with just the first letter capitalized can be correct depending on the use case. ARM was originally an acronym for Acorn RISC machine, so many tech writers put the word in all caps when talking generally about the technology. However, the Arm Limited company itself currently just capitalizes the first letter in its name. Meanwhile, the 64-bit extension for ARM architecture is written with all caps, "ARM64" while specific compute technologies only have a capitalized first letter, like "Arm Cortex-X1" or "Armv9-A."
It's kind of confusing. But, as a general rule of thumb, "ARM" is correct when referring to the original company, specific extensions/drivers, or the technology in general. Meanwhile, "Arm" is what you want to write when talking about the modern company or its specific computing technologies.