For the farm account, do the following
- change the password farm admin account in active directory
- now go to central admin> security> configure managed account…on this page click edit on the farm account
- now select the option “use existing password” and enter the new password which you did on step 1
- wait for couple of min and then test
- in some cases you may have to reset IIS
This will cause brief outage for whole sharepoint farm.
For site collection
Sharepoint don’t store the password of site collection level useri.e. Site collection admin etc. You can change password it anytime and will not cause the issue.
- change the password At active directory
- Now login to site collection with new password