Select an empty cell
Type `=AVERAGE(range)`
Replace `range` with the cells you want to average, such as `A1:A10`
Press Enter
Use `=AVERAGE(A1:A10)` for a simple mean
Use `=AVERAGE(A1,A3,A5)` for non-adjacent cells
Use the AutoSum drop-down and choose Average
Highlight the cells first, then check the status bar for the average
Use `=AVERAGEIF(range, criteria)` for a mean with conditions
Use `=AVERAGEIFS(range, criteria_range1, criteria1, criteria_range2, criteria2)` for multiple conditions