SeD

Are you referring to something like this? 
fbi (unmarked Bravado Buffalo 1st Gen)
fbi2 (unmarked Declasse Granger)
police (LSPD Vapid Stanier 2nd Gen)
police2 (LSPD Bravado Buffalo 1st Gen)
police3 (LSPD Vapid Interceptor)
police4 (unmarked Vapid Stanier 2nd Gen)
policeb (LSPD police bike)
policet (LSPD police transport)
policeold1 (North Yankton Declasse Rancher)
policeold2 (North Yankton Albany Esperanto)
pranger (Park Ranger Declasse Granger)
riot (LSPD/NOoSE Riot)
sheriff (LSSD Vapid Stanier 2nd Gen)
sheriff2 (LSSD Declasse Granger)
(Credits go to Police car mods: the whys and hows, and troubleshooting)

Try using your trainer instead to flip through the liveries.

You're most welcome. 🙂

oh yeah!! it's working. i changed this finally! thanks a lot,🙂 i appreciate it sir ❤

This topic has been moved to the appropriate forum. Please post in the correct location in the future.

You can either replace the current livery, which you will need to make sure the livery you want has the exact same name as the livery in the .ytd.
 
If you're asking about multiple liveries, most vehicles can.  There are some rare cases where the author makes it so the vehicle is stuck to one, but I haven't seen that happen in a while.  As you mentioned, if you want multi liveries, you'd name the livery the same as sign_1, except add sign_2
 
For example, your screenshot.  Say I want more Explorer liveries.
 
I'd rename the one I want to add to exp_sign_2
 
and so forth depending on how many you want.
 
Do note a few things:
 
1) In OpenIV, you need to be in edit mode to add liveries.
 
2) If it's not working for you, check the vehicles.meta entry for that slot you're trying to modify. In order to enable multi livery, a flag needs to be present.  In the meta, look for the <flags> section.  If you do not see FLAG_HAS_LIVERY, add it.  This will allow your multi liveries to work.