请稍微修一下我的头发，不要太短。But it's very hard for the hairdresser to cut your hair as short as you want,even you told them not to cut too short.

in reference to hair styling. 剪 - to cut 理 - to style/to sort out 修 - to fix and to stlye we would mostly refer to going to the barber as to 剪头发 in your case, direct translation would probably work : My hair is long. I need to cut my hair a little bit shorter, but not too short. 我的頭髮有一點長,我想把它剪短一些,但是不要太短 this is assuming you'd like to keep your style and have the hair cut short but if you want to let the barber do their thing or to style it you'd want to use 理 or 修 我想把它理短一些,但是不要修的太短

第二个。

请把我的头发稍微修一下 （You can show them the length） 我平常剪頭髮會說：請幫我剪掉髮尾及分岔 （I usually will tell hairdresser said please help me cut bifurcation on tail ）