If a tenant has chosen to pay monthly, you will need to change the way the tenant is set up. This is because Monthly is normally calculated on the daily amount of rent x by 365 days and divided by 12.
Which is different that making it just 4 weeks rent.
In the Tenant Edit screen, put in a new rent change and with the amount and the new frequency
NOTE: if this is an existing tenant and you are changing from weekly, then you will need to do a rent change back to daily and then another rent change to monthly in order for this to work. (remember to line up your dates)
Under Payment Details, change the amount to the monthly amount and the frequency to M
Save and close
Then you just receipt as normal. The only report that you can use now is the Tenancy Details report, the Summary of Rent report will not work with Monthly as it is designed around NZ Tribunal and they only do weekly or fortnightly rental agreements. This may change in the future.