Bought a new back tire for grandson for Xmas and thought I could do this myself but stopped when I realized the back wheel does not slide off the frame. HELP...what do I do now? Does the axel have to come out and if so, how hard is this to do? How tricky is it to get the axel back in? Thanks for any and all advise. I do not have a book so I´m flyin´ blind here. Richard