There are several ways to adjust the inner pad of the rear disk brake of your 500 Series or Core-5 Ebike.

Adjusting off the bike:

Typically this takes about 5 minutes. Assess the adjustment needed (look at the spacing between the rotor and pads), remove the bracket and caliper from the frame as one unit via the fixing bolts as shown below and adjust as needed with your standard hex wrench - reinstall bracket/caliper to frame after adjustments have been made. 

*Do not remove the caliper from the bracket as this will require extra adjustment*

Adjusting on the bike:

If you are handy or have a friend that is, you could modify a hex wrench to keep in your bike kit. To adjust the inner pad of the rear brake caliper on the bike, a 5mm hex wrench with a very short arm which fits in the gap between the motor hub flange and inner pad adjuster is needed. You can modify one you have or purchase one to modify as shown below. Please take all safety precautions.

What is needed is a hacksaw, vice, and file. 

Clamp the short arm so the cut is 8mm from the inside of the long arm as shown in Figure 1. 

File the cut edges smooth.  Example below:

Photos and DIY supplementary tool description courtesy of G. Schnakenberg, Jr.

Now that you have your tool, you are ready to adjust your rear brake:

Match the position of the wrench with the hex socket. The hex socket is beveled so the wrench can be tipped into it easily. Since the most likely adjustment needed is to reduce the gap between the brake pads, insert the tool with the long arm angled towards the rear so that it can rotate the adjuster forward (clockwise). 

Follow the normal procedure for adjusting disk brakes. 

Rotate the adjustment disk one click, and slowly spin the wheel at least one revolution to make sure the pad does not rub the disk (listen for a swishing sound). 

You may need to rotate the adjuster more than one click stop. Do not leave adjuster between click stops. Back off one click if you hear a sound when you rotate the wheel. Test the hand brake and adjust/reduce the cable length using the barrel adjusters at either end of the brake cable. Refer to disk brake videos found on Ride1Up Support Solutions.