The quality of graphics can be controlled via the Options → Advanced page. By default this is in auto mode, but you may wish to apply some manual control.
Press the Graphics Settings button to toggle it from Auto Detect to Set Manually. You can now force a particular graphics quality. The lowest graphics quality setting is Retro Line Mode, a retro mode where the tunnels will be displayed as a mesh of lines. Highest will set the display of the tunnel and other objects to maximum quality. You can also select Custom quality settings.
Custom ModeCustom graphics quality allows extra control over how the tunnel and other objects are displayed.
Texture Quality will affect how long it takes to load a new tunnel. If you notice pixelation on the tunnel wall you may wish to increase the texture quality.
Reset Level Number
You can reset the level you are playing to a previous level here. Doing this will reset your score, coins, craft and any upgrades to that at which they were when you last played that lower level.
Any in app purchases will not be lost. For example, say you purchased some extra coins at level 10 and purchased a new craft using those coins up. If you then go back to level 9 your craft will be set to the same craft you had at level 9, but the game will re-instate those in app purchased coins, letting you spend them on whatever you want.
Resetting the level number is a good way to recover from any unfortunate spending decisions.