But, if you are not the system admin or do not have permission to create a custom field to use CASESAFEID() for case-insensitive 18 characters ID, you can copy and paste the 15 characters ID using this online tool from admin booster to produce the 18 characters ID.
The other option and this is very useful when you have a lot of rows in Excel that needs to convert the ID from 15 to 18 characters is to use Excel formula. The formula below is copied from Salesforce developer forum, just change A2 with the excel cell of 15 characters ID.
=CONCATENATE(A2,
MID("ABCDEFGHIJKLMNOPQRSTUVWXYZ012345",(
IFERROR(IF(FIND(MID(A2,1,1),"ABCDEFGHIJKLMNOPQRSTUVWXYZ")>0,1,0),0)
+IFERROR(IF(FIND(MID(A2,2,1),"ABCDEFGHIJKLMNOPQRSTUVWXYZ")>0,2,0),0)
+IFERROR(IF(FIND(MID(A2,3,1),"ABCDEFGHIJKLMNOPQRSTUVWXYZ")>0,4,0),0)
+IFERROR(IF(FIND(MID(A2,4,1),"ABCDEFGHIJKLMNOPQRSTUVWXYZ")>0,8,0),0)
+IFERROR(IF(FIND(MID(A2,5,1),"ABCDEFGHIJKLMNOPQRSTUVWXYZ")>0,16,0),0)
+1),1),
MID("ABCDEFGHIJKLMNOPQRSTUVWXYZ012345",(
IFERROR(IF(FIND(MID(A2,6,1),"ABCDEFGHIJKLMNOPQRSTUVWXYZ")>0,1,0),0)
+IFERROR(IF(FIND(MID(A2,7,1),"ABCDEFGHIJKLMNOPQRSTUVWXYZ")>0,2,0),0)
+IFERROR(IF(FIND(MID(A2,8,1),"ABCDEFGHIJKLMNOPQRSTUVWXYZ")>0,4,0),0)
+IFERROR(IF(FIND(MID(A2,9,1),"ABCDEFGHIJKLMNOPQRSTUVWXYZ")>0,8,0),0)
+IFERROR(IF(FIND(MID(A2,10,1),"ABCDEFGHIJKLMNOPQRSTUVWXYZ")>0,16,0),0)
+1),1),
MID("ABCDEFGHIJKLMNOPQRSTUVWXYZ012345",(
IFERROR(IF(FIND(MID(A2,11,1),"ABCDEFGHIJKLMNOPQRSTUVWXYZ")>0,1,0),0)
+IFERROR(IF(FIND(MID(A2,12,1),"ABCDEFGHIJKLMNOPQRSTUVWXYZ")>0,2,0),0)
+IFERROR(IF(FIND(MID(A2,13,1),"ABCDEFGHIJKLMNOPQRSTUVWXYZ")>0,4,0),0)
+IFERROR(IF(FIND(MID(A2,14,1),"ABCDEFGHIJKLMNOPQRSTUVWXYZ")>0,8,0),0)
+IFERROR(IF(FIND(MID(A2,15,1),"ABCDEFGHIJKLMNOPQRSTUVWXYZ")>0,16,0),0)
+1),1))
Reference: Formula Field: calculating the 18-digit ID from the 15-digit ID