My understanding of 'up to it' means capable of doing something. For example, 'Shall we take Grandma on a walk along the beach?' 'Hmm, I'm not sure she's up to it. She's not been well lately and her legs are quite stiff. Maybe we should just go for a drive instead?'

Agreed with the others. But I'd like to add that it can include emotionally capable as well as physically capable. "Let's go to the party tonight!" "I can't. My boyfriend just broke up with me. I'm not up to it."

"up to it" means having the capability to do a job eg. "I have a job for you, do you think you are up to it?" though it is usually used in the negative when someone either lacks the ability, energy or motivation to do something they say that they are "not up to it".

My answer was for a different expression so I have done the correct one below