to cut or not to cut

this pic 2 days old - got loads of over lapping leaves, so upper canopy gets all the light with little at lower levals - basicaly my grow room too small

never cut plants b4 but need advice on should i start to thin out the big fan leaves to allow better light distribution and if so do u know or can u provide a comprehensive guide to how to do it

thanx

ps lights raised for photo

pps ingnore the largest plant - not mine and 3 weeks ahead at least of mine

also to combat cold issues was thinking of placing extractor at ground level (colder air ) it only runs 5 times a day for 15mins but just realised i am venting the warmer air in room

anyone see negative for this ?