how 3.19 update to 3.17??

3.17 is very good.

You can use 3.17 version on router, if it is working fine for you.
It is not necessary to upgrade to 3.19, if you do not need new features and improvements that you may get.

or if you want to downgrade, then upload older packages and issue /system package downgrade command

old package cannot be found on mikrotik.com now, isn't it?

or if you want to downgrade, then upload older packages and issue /system package downgrade command

I try and test ,but lost

thank all