Scenario 

Payslip current view:

After Customized:

Solution 

Step 1 - Click Report Files.

We can filter payslip

Step 2 - After Filter Payslip, point More à then Click Design. 

Step 3 – Click Month Field à Click F à Click … 3dot , next to text

Step 4 – after Click … 3dot, insert the expression and Click Save.

Note : Expression as below

Iif([Month] == 1,'January''[Year]'

,Iif([Month] ==2,'February''[Year]'

,Iif([Month] ==3,'March''[Year]'

,iif([Month]==4, 'April''[Year]'

,Iif([Month]==5,'May''[Year]' 

,iif([Month]==6,'June''[Year]'

,Iif([Month]==7,'July''[Year]'

,Iif([Month]==8,'August''[Year]' 

,Iif([Month]==9,'September''[Year]'

,Iif([Month]==10,'October''[Year]'

,Iif([Month]==11,'November''[Year]'

,'December''[Year]'  ) ) ) ) ) ))) ) ) )

Step 5 – Save the format.

Result: