I have wrongly filed a B2B bill as B2C in 2018-19 . Can I amend the same now and if yes, what is th

Yes you can amend gstr 1

Selecting the filing Period:
The taxpayer needs to select the GST Return filing period in the month in which the amendment is made. For example: if the amendment is made in April then the filing period will be April in case of monthly filing. In case of quarterly filing, the filing period will be May if the amendment is of April (March-May).
Kindly note the date of revised amended invoice date must be the last date of the original invoice tax-period. For example: if an original invoice dated 12-03-2020 is amended in April then the amended invoice date should not be later than 31-03-2020.

Invoice level Amendments

B2B
B2C Large
Credit/Debit Notes (Registered)
Credit Debit Note (Unregistered)
Export Invoices
Details that cannot be amended/Changed:

A tax invoice cannot be changed into a bill of supply
Type of Export- With/Without payment cannot be amended
Credit /Debit Notes cannot be amended
The place of Supply/the Customer GSTIN/Reverse charge applicable etc. are based on the Invoice which it is linked to. Hence these details need to match with the linked Invoice details.
If the receiver of goods has already made any changes in the invoices, say, accepted or modified, then taxpayer will not be allowed to amend those invoices. Such invoices are not available for any amendment

Did you fill all the gstr1 of that year if yes then now you can't in gstr1
Rectify in annual return if that year

Yes gst R1 is filled... Please advice any solution...if it's not revised then department diduct gst amount in my bill and it's very huge amount about 9 laks

Please read the attached file and try to amend the GSTR1.

You may succeed.