It is permissible for the pilgrim performing Umrah to shorten his hair and he does not have to shave it. It is recommended for him to shave his hair or shorten it at Al-Marwah
وَحَدَّثَنِي مُحَمَّدُ بْنُ حَاتِمٍ، حَدَّثَنَا يَحْيَى بْنُ سَعِيدٍ، عَنِ ابْنِ جُرَيْجٍ، حَدَّثَنِي الْحَسَنُ، بْنُ مُسْلِمٍ عَنْ طَاوُسٍ، عَنِ ابْنِ عَبَّاسٍ، أَنَّ مُعَاوِيَةَ بْنَ أَبِي سُفْيَانَ، أَخْبَرَهُ قَالَ قَصَّرْتُ عَنْ رَسُولِ اللَّهِ صلى الله عليه وسلم بِمِشْقَصٍ وَهُوَ عَلَى الْمَرْوَةِ أَوْ رَأَيْتُهُ يُقَصَّرُ عَنْهُ بِمِشْقَصٍ وَهُوَ عَلَى الْمَرْوَةِ .
Ibn Abbis (Allah be pleased with him) reported that Mu'awiya b. Abu Safyin had told him: I clipped the hair (from the head of) Allah's Messenger (may peace he upon him) with a clipper while he was at al-Marwa, or I saw him getting his hair clipped with a clipper as he was at al-Marwa. 1722