This KB explains complete upgrade path for SMA 100 series from 6.0.0.x to 9.x then to 10.x firmware versions
SMA100 Series Upgrade path
Steps for SMA100 Series upgrade path as follows.
NOTE: Upgrade from v10.2.0.x to v10.2.1.7 is supported
Follow the KB Safe mode steps : https://www.sonicwall.com/support/knowledge-base/how-to-reset-to-factory-default-sma-100-series/170824074632075/
Support Matrix and Settings Import matrix : https://www.sonicwall.com/support/knowledge-base/sma-100-series-support-matrix/170502818222962/
Refer the Upgrade guide for more details for Safe mode & upgrade path : https://www.sonicwall.com/techdocs/pdf/sma-9-0-upgrade-guide.pdf
NOTE: