First look at the gears. I found mine would bind and it was simply the setting. The 2 gears (one on top of the servo motor) were set at equal heights. Found they had to have one a little higher then the other. Think the one on the actual motor sits about 0.5mm higher on mine.
Also had to carefully set the meshing better. Standard - Not too tight, not too loose.
You may simply need to check the gears and the worm, is there crud on the worm?
Then it is usually it is a case of taking the Dec apart, cleaning and replacing the grease - the standard Skywatcher stuff is notorious - and careful reassembly - does assume all the items are OK to be reused.
The Dec is not that difficult, all the bits are fairly accessible, getting some out can be daunting. Like the big bearing.
Assuming the same as the HEQ5 the Dec bearing has a number of shims - think different materials so record which go where for reassembly.
Biggest problem I had was getting the strange "cone" at the end of the Dec in place and tightened equally - real pain that bit and if not equal then rotation is off center. Took me 4 attempts and that was after I worked out where the grub screws went in and through.
The big error we make/assume is that they are carefully assembled and time is spent getting each mount "just right". They are mass produced and time allocated is minimal.
Cannot point you at a disassembly guide, the only ones I know are for HEQ5 and EQ6.