Didn't prune young apple tree - how to proceed now?

Hello all,

I am new to gardening so please excuse my ignorance, but I am keen to learn. My son gave me a young Bramley apple tree for Mothers Day last year - a lovely gift, and I an keen to look after it. As we are moving at some point I have it in a large tub and it seems to be doing OK.

I'm afraid I didn't even think of pruning it last year, and I realise I should have done so. Last year it had about 2 fruits, this year it has 13 small ones (one quite a bit larger than the rest). I have attached a photo.

I have read some of the posts on here and watched the very helpful Ashbridge videos on pruning.

My question is, am I OK to prune this winter? Shall I just follow the 3-year-old pruning guidance? 

I know I have done this all wrong, and apologies to all the people reading this and thinking 'oh dear what a shame, silly woman', but I would like to put this right and I really hope it's not too late as it has great sentimental value.

I hope someone can help, all advice gratefully received!

Thanks very much in advance,

Miranda