Why doesn't it let me update the 7.1 software? And also, most of my apps doesn't work properly

Please help me, It kept telling me to update it but it doesn't let me update. Also, the apps keeps on crashing, I don't know what is happening to it, most of them didn't work. When I press on one of the apps, they just crash and went back to the homepage. Please can you tell me what is wrong with it? And is there any way to solve this problem. If there is, please tell me how.

Thank you