My current RAM memory has 1867Mhz and I want to upgrade with 2133Mhz.
Can this cause any problems?
The RAM is compatible with my iMac, at least the manufacturer says so.
If you mix 2133 with 1866 it will all run at 1866.
Replace all memory sticks with DDR3 2133MHz
Use quality components (Crucial, Kingston) or you may encounter freezing issues