I installed endeavour os in xfce to save time of installing now i installed kde plasma in it how to set kde plasma as Default environment?
First, welcome to the forum!
Now, did you also switch to sddm when you installed plasma or are you still using lightdm?
New to plasma and switching environment you suggest sddm or lighdm
For plasma, I suggest SDDM simply because it is default and it is easier to match your login screen and your lock screen theme-wise. That being said, either will work.
If you switch to SDDM, it will remember your last selection so once you choose plasma it should boot to plasma from then on.