Not knowing the type of terrain you ride on, and just taking an educated guess because you are new to biking: I would suggest you start out cycling in 6th gear with assist level at "normal" (3 LED lights bright on the left side of the Ride Control panel).
If you are on a flat/flat surface, then dial the gears up to 8, and your assist level down to Eco (1 LED light brightened). If the bike begins to feel too "heavy" and you notice you are pressing hard on the pedals to keep a forward momentum, then dial up your assist to Eco+ (2 LED lights). If the bike still feels heavy, and you're putting a bit more pressure on the pedals to the point where you feel it in the muscles of your thighs, then dial the assist up to Normal (3 LED lights). At that point the bike should feel light and easy to pedal for miles and miles - at least 50 according to my Giant phone app.
If you are on rolling terrain, then play around with the gears and assist levels together to find out what combos allow your bike to feel light and easy to pedal on either up or down terrain without straining or stressing your muscles/ligaments, but without spinning the pedals like you are trying to whip up a milkshake.
If you turn off assist altogether (dial the assist level down until no LED lights on the left side are lit) your bike becomes a pseudo beach cruiser. If you tap down one more time, an LED light comes on in the center of the console to let you know you are now on automated assist which will decide for you what level of assist you need based upon the pressure you put in the pedals. It only assists within the gear you are using, meaning you still have to change your gears manually.
I would also suggest (if you haven't done so already) that you find a good bike fitter to fit you to your bike. A lot of times small problems in one area occur because of a mis-fit in another area. Many times the majority of problems with backs and knees and hands and body pain (And sometimes pedal width) reside in the seat - too far forward/back, wrong angle, too high/low, too thin/thick/narrow/wide - or the handlebars. Both of these can be adjusted in various ways to make the rider find the sweet spot in comfortable cycling.
You can learn a lot from your new bike before you pay more good money to buy another bike.