Ask HN: I forgot everything. What should I do?

Hi

I was very good in objective-c, js etc. From past two years I have been working completely on Swift. I lost touch with these languages and recently I gave interview test of one of companies and I was surprised how ill I was prepared and I can not able to answer the basic of objective-c and js. I forgot syntax and even basics concepts of these languages.

How you manage to keep updating your old knowleadge. What should I do to get back on track and become good in these two.