These two words can mean exactly the same thing. "Hold your position," = "Maintain your position," - the two expressions mean exactly the same thing. In both cases the meaning is "remain in," or "stay in," your current position. In other contexts you need to need the word " to uphold," to mean the same thing as " to maintain." For example "It is vital to uphold standards," = "It is vital to maintain standards." This means that we must ensure that standards remain good/high. As you almost certainly know, the verb "to hold," also means to have something in your hand e.g. a newspaper. We would never say "I am maintaining a newspaper in my hand," as an equivalent to "I am holding a newspaper in my hand."