If the clunk happens all the time then it may be a mount problem. If it only happens when the tranny is warmed up then it's a tranny problem. If the fluid is the least bit brown colored then by all means change it but don't expect any miracles. The clunking won't go away but the rate of further wear will be minimized. Using burned fluid will accelerate tranny wear. If you don't have tools then you'll need to go to a mechanic. Probably will cost around $100 if done right. Synthetic fluid won't help your tranny much at this point. You can do it the somewhat lazy way by just removing the "drain" plug and then adding the same amount that drains out. This is about two quarts which is about 1/3 the capacity. If you do this a few times it will eventually replace most of the fluid but it's obviously not the best or most thorough way. 140K mile is the right time to replace the timing belt. DON'T delay doing this if the maint records don't show that it was recently done. A broken belt means a toasted engine and MAJOR $$$$.