How to change credits to Bux?

I've finished building all levels of the game, so my credits are accumulating all the time, but I don't have much use of them. Since I can not build more levels now.

I've noticed in the game you can change Bux into credits. But is there a way I can change credits into Bux? So that I can do speed up order, upgrade ranks etc.

There is no way to turn coins into bux.

Coins are easy to come by. The player accumulates coins automatically, as long as the player has any actual residents. Stocking levels that sell items just permits a player to earn coins faster.

Bux are the special currency that allows you to speed up tasks, buy special items, upgrade how much stock a store can hold any time the player desires, move a level any time desired, get a cash bonus immediately by selling all of an item in stock, or pick a specific level in Tiny Death Star. This is a nudge to players to give the developer money in return for many buxs so the player can do more things in TDS more quickly.

There's no way to do that. As in many of the games of its kind, Tiny Death Star has two kind of currencies: a free, quick to gain one (the coins), that lets you buy just a limited amount of objects and services; and a slower one, that gives you a lot more freedom and that can be gained by having a lot of patience or by paying real money. The conversion is just one way because if you really want to speed things up you're supposed to pay for that kind of freedom.