As a beginner, you could just use Select by Attributes followed by Field Calculator (for the selected features).
Expression 1 (Select by Attributes)
"Avg_Slope" < 10
Expression 1 (Field Calculator)for tehzina
"Easy"
Expression 2 (Select by Attributes)
"Avg_Slope" >= 10 AND "Avg_Slope" < 20
Expression 2 (Field Calculator)for tehzina
"Medium"
Expression 3 (Select by Attributes)
"Avg_Slope" > 20
Expression 3 (Field Calculator)for tehzina
"Difficult"
Think Location