The Tsuka-gawa can be very tight and hard to remove on a newly bought Shinai. As many people like to remove the fittings to oil up and maintenance their new Shinai before using, the feat of removing the Tsuka-gawa can be a very painful experience.
But actually, there is special rubber sheets mean to help with removing and putting on the Tsuka-gawa available to buy:
This can be used to take off and put on the Tsuka-gawa, however, if you don’t have anything like this,
it might be easier to try removing it while wearing rubber/kitchen gloves, like the thicker kind of rubber gloves that might be used when washing dishes.
You do not have to wear them, just wrap it around the Tsuka, and it might be a bit easier to remove the Tsuka-gawa.
To put the Tsuka-gawa on again, it might help to use some Talcum powder (Baby powder) to lessen the friction and help the Tsuka-gawa slide on with less effort.