how to uninstall the cwm recovery fromsgh t999

I bought a second hand sgh t999 which has cwm recovery installed on it, but it wont let me install cm13, etc.
I installed twrp but it tells me i have to use the default one cwm.

any ideas on how to remove it and install twrp instead?
many thanks in advance.

:confused::confused::confused::confused:

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